TK 3201 
. S45 
Copy 1 




Electrical Record Series 
of Practical Information 


PROFITABLE 

POWER WIRING 


THE GAGE PUBLISHING CO., Inc. 
461 Eighth Avenue New York, N, Y. 
























PROFITABLE 
POWER WIRING 

W: J. SHORE 

/• 

Contracting Electrical Engineer 


THE GAGE PUBLISHING CO., Inc. 
461 Eighth Avenue New York, N. Y. 


K 3 e 0 ' 
,S45 


Copyright 1924 

The Gage Publishing Company, Inc. 
New York 


HAY -5 *24 

©cu79 2253 



^PO { ( 



CONTENTS 


r 

Vp 

V"N» 


The Design of Power Wiring Systems (Part I). 1 

The Design of Power Wiring Systems (Part II). 21 

Buying and Installing Panel Boards. 27 

How to Figure D. C. Power Wiring..—. 37 

How to Analyze Load Preliminary to A. C. Power 
Wiring Work . 57 

How to Select the Correct Type of A. C. Motor. 69 

How Many Power Panels Should Be Used?. 89 


Economy of Grouping Motor Circuits in One Conduit—111 










The articles composing 
“Profitable Power Wiring” were originally 
published in Electrical Record. 


CHAPTER ONE 


The Design of Power Wiring Systems 

PART I 


T HE average electrical contractor has had more 
experience in the installation of wiring for light¬ 
ing purposes than he has had in the design and instal¬ 
lation of electric wiring systems for power applica¬ 
tions. 

One of the reasons for this is apparent in the fact 
that 90 per cent of electric wiring is installed for light¬ 
ing and only 10 per cent for power. Another reason 
is that wiring for power requires more skill, experi¬ 
ence and knowledge, and the average contractor is, 
therefore, not so keen to follow up work which takes 
him out of his daily duties and which, besides, re¬ 
quires so much extra time and effort to prepare plans, 
specifications and estimates. 

This book or chapter is, therefore, written for those 
contractors who have not done much power work and 
who are anxious to learn something about its elemen¬ 
tary principles in order that they may broaden the 
scope of their activities and share in some of this 
business which, although more difficult, is varied, inter¬ 
esting and profitable to handle. Every job is different 
and every job is new. 



2 


Profitable Power Wiring 


Those who like to handle machinery and see the 
machines turn raw materials into finished goods will 
find a charm and a fascination in the design of elec¬ 
tric power wiring systems that will hold them to the 
business for a long, long time. 

For the purpose of this chapter it has been assumed 
that the contractor’s customer has ordered his motors 
from the contractor, who with the help of the motor 
manufacturer’s representative, has made recommenda¬ 
tions regarding the number of motors to be furnished, 
their respective sizes, and their various special charac¬ 
teristics, including speed, type,' winding and other 
equipment such as starters and control apparatus. 

It has been definitely determined just what work 
each motor will do and just where it will be placed. 

At this point it becomes the duty of the contractor 
to prepare specifications for a power wiring system 
which will secure the best possible operating service 
and at the lowest first cost. 

Aside from mechanical considerations such as neat 
workmanship, rigid suspension of conduit and cables, 
well soldered joints, the following electrical elements 
are desirable in any wiring system for power: 

1. Ample conductivity of conductors. 

2. Proper overload protection of feeders and sub¬ 
feeders. 

3. Proper overload protection of motor circuit 
wiring. 

4. Proper overload protection of motor apparatus. 

5. Facilities for the installation of additional appa¬ 
ratus at any location at reasonable cost. 

If the motors are properly selected for their work 
and all of the above conditions are present, the net re¬ 
sult will be an installation that should run for years 
with little or no electrical trouble to distract the cus¬ 
tomer from turning out his product in ever increasing 
quantities. 


The Design of Power Wiring Systems 


3 


Simplest Way Not Always Best 

To design and install such an apparatus is a nice 
piece of work requiring skill and knowledge, but to 
secure such an installation against competition requires 
more skill and more knowledge and it is the intention 
of this chapter to point out some of the underlying 
principles and to work them out in detail so that their 
application will be readily understood and so that they 
can be used on new power jobs that come up from day 
to day. 

The simplest way, although by no means the proper 
way, to wire for a number of motors is to run a sepa¬ 
rate line and circuit from the service to each motor. 

Here is an actual instance. An iron works started 
to electrify its plant and originally put in ten motors 
aggregating 150 hp. A separate line was run for each 
motor. As business developed and new machinery 
was added, more motors were installed until at one 
time there were twenty motors, and twenty separate 
circuits were run from the service. Each time a 
new motor was added, a new cut-out was mounted 
near service and new splices were made. The main 
fuse block became gradually overloaded, -nd trouble 
developed. An overload on any one motor would 
cause a blow-out of the main fuses and the whole 
plant would be shut down. With the great amount 
of splices and soldered joints, weak points showed up 
under heavy load and continuous steady service was 
out of the question. 

Then, every time it was necessary to add a new 
motor to the plant it was necessary to run a new sepa¬ 
rate line, sometimes as great a distance as 600 ft. The 
cost of wiring was several times that of the motor 
cost. The climax came, however, when it was neces¬ 
sary to add eight J^-hp. motors. There was neither 
room nor space at the main line service for more cut¬ 
outs and the cost of running extra lines was very high 


4 


Profitable Power Wiring 


indeed in proportion to equipment costs. It wa. there¬ 
fore decided to install a main distribution panel, with 



Instead of Bringing Wires from 
Service Entrance Each Time a 
Motor Was Installed, This Main 
Feeder Panel Was Used 


feeders, and sub-distribution panels. In the above 
illustration is shown the new feeder panel in place 
right next to ihe entrance of the service. 

Besides running a separate and individual circuit 
direct from the service to every motor, there is another 
system that has been used quite extensively. 

This method consists of one main feeder run 
throughout the factory, and at various points along 
its length are installed combination pull boxes and cut¬ 
outs. A tap is taken direct from the main conductors 
and connected to the cut-out in that box to feed a 
motor right in its vicinity. This method is a cheap 











The Design of Power Wiring Systems 


5 


manner of installation and carries with it a number 
of disadvantages which often more than outweigh its 
low first cost. 

The most serious disadvantage is that the cut-out 
boxes are always on the ceilings. It is therefore 
always necessary to use a ladder or other rig to test 
and replace fuses. Sometimes it is necessary to do 
this where there is shafting and belting and revolving 
pulleys with the resultant hazard. The lack of accessi¬ 
bility to fuses is a great detriment. Making splices in 
these pull boxes on the ceiling is difficult. The rub¬ 
ber insulation on conductors is seriously affected and 
weakened. When a new motor is added it is neces¬ 
sary to cut the feeder and insert a new pull box, an 
expensive affair and not so satisfactory. 

This system is not recommended except in excep¬ 
tional instances which will be described further on. 

The reasons why distribution panels, sub-distribu¬ 
tion panels and feeders and sub-feeders are used in 
connection with electric motor wiring are several. 

They reduce the cost of conduit, wire and 
labor. 

They secure better electrical service in that 
a breakdown or trouble in any section will not 
affect the rest of the equipment. 

They provide easy facilities for expansion 
and the use of more equipment. 

Every feeder running to a distribution panel may be 
designed to carry an excess of 10 per cent over the 
connected horsepower with an additional expense of 
only 2 to 3 per cent. Every panel board may be pro¬ 
vided with an extra fuse gap to allow for an extra 10 
per cent additional load at an expense of only 2 to 3 
per cent. And very frequently it is found that owing 
to the diversity factor and the demand factor the 
feeder can carry more load than it was originally de¬ 
signed for. 


6 


Profitable Power Wiring 


By diversity factor is meant a condition where all 
the connected motors are not operating at the same 
moment. By load factor is meant a condition where 
all the connected motors are not operating at their 
full load part of the time or all of the time. 

Thus it is evident that in many instances feeders are 
capable of carrying a greater load, whereas if individ¬ 
ual circuits were used, there would be no such leeway. 

It must be borne in mind, however, that this condi¬ 
tion does not always apply, for in some instances such 
as flour mills or cement plants, the load is continuous 
and at its maximum all of the time. 

It is often far more difficult to describe and explain 
how to do a piece of work than it is to go ahead and 
actually do it. The contractor who has specialized on 
the design and installation of power wiring systems 
finds no difficulty in laying out and doing such work, 
but, if you say to him, “Why did you put in three 
power panels, and why didn't you put in five panels ?” 
like as not he would say: “That is the best way to 
do it.” If, on the other hand, you ask the recommen¬ 
dation of the panel board manufacturer, you would 
probably have 2 or 3 more panels than you actually 
needed. 


Service Entrance Information 

Most of the wiring done will be supplied with elec¬ 
tric service from the local electric light and power 
company, and therefore a few words in regard to that 
phase of the matter would perhaps be useful. Where 
the customer generates his own supply of electricity, 
this information would not be so pertinent. Befoie 
commencing to lay out any job it is necessary that 
you go to the local central station and get in touch 
with someone who is able to give the location at which 
their wires will enter the building. 


The Design of Power Wiring Systems 7 

In some instances the central station does all the 
service work, in others it requires the customer to do 
this work, and the company only supplies and installs 
the necessary metering equipment. Sometimes wires 
will come from an underground service and at other 
times it will be an overhead service. Sometimes 
service will be low tension 220 volts and at other times 
2200 or 7800 volts. In some instances the contractor 
has nothing to do except to bring his main cut-out to 
the service entrance, and at other times he has to put 
up an outside standpipe with his conductors extending 
therefrom, has to furnish a meter cabinet, a main 
service and test switch, lo mount meter transformers 
and to wire it up completely. 

In any event the contractor when he meets the com¬ 
pany’s representative on the job and tells him what he 
wishes to accomplish should be certain to get all de¬ 
tails and locations and instructions before permit¬ 
ting him to depart. 

He should not attempt to cut corners and try to save 
on expenses. For the central stations, as a rule, do 
not approve exceptions and they insist upon strict ad¬ 
herence to their rules and regulations. Anv variation 
from these is apt to delay the connection of service 
and will result in inconvenience and delay to the cus¬ 
tomer and will mean extra expense to the contractor, 
who will have to come and fix it up correctly. A good 
rule to follow is, “Do it right the first time.” 

Securing written approval of specifications and 
sketches will help greatly to avoid misunderstanding. 

If, for example, we had a factory whose plant con¬ 
sisted of several buildings as shown in the sketch, and 
electric service entered in the small building A, and 
there were 30 hp. installed in Building B, 40 hp. in 
Building C, and 60 hp. in Building D, the simplest and 
most logical method of wiring would be to put a dis- 


8 


Profitable Power Wiring 


tribution feeder panel in A and run one feeder to 
B to carry 30 hp. and one direct to C to carry 40 hp. 



and one to D to carry 60 hp. When we get into each 
of the buildings we would again analyze our conditions 
and lay out the wiring system still further. 

The first thing to do in laying out this kind of work 
is, metaphorically speaking, to lay it out before you 
so that you can get a bird’s eye view of it and can, at 
your desk, look the thing over and not be distracted 
by the noise, dirt, confusion and many other things 
that are taking place in the factory that you intend to 
wire. 

A simple and convenient method that takes but little 
time is to prepare a sketch of the floor plan of your 
building. Almost every factory floor space is evenly 
divided into a number of bays. Measuring the length 
and width of one or two bays will give you dimensions 
of the entire factory. Count the number of bays 
lengthwise and then crosswise. Draw a rectangle or 
square, or any shape to correspond with the factory 
floor plan. If it is three bays wide, subdivide your 
sketch lengthwise into three divisions. If it is eight 
bays long, divide your length first in two, then each 
section in two and then each of the remaining sections 
in two. Where the lines cross, draw little squares to 
represent posts. Having measured the length and 















The Design of Power Wiring Systems 


9 


width of each bay, multiplying length by eight and 
width by three will give you outside dimensions. 

Next, plot in each of these bays the motor or motors 
that are to go in. These can be shown by a little circle. 
Number the circles consecutively and in the margin 
specify the data corresponding to that motor, its horse¬ 
power, its speed, winding, whether floor mounting or 
ceiling suspension as follows: 









I 




'a 1 



w 1 

* (9 





'a J 

' 1 


ij 


1— 5 H.P.—1800 r.p.m.—Shunt—Ceiling 

2— 10 H.P.—1300 r.p.m.—Comp.—Floor 

3— 7y 2 H.P.— 850 r.p.m.—Comp.—Floor 

4— 2 H.P.—1200 r.p.m.—Shunt—Ceiling 

5— 15 H.P.— 600 r.p.m.—Comp.—Floor 

6— 1 H.P.—1700 r.p.m.—Comp.—Ceiling 

By so doing this preliminary work you accomplish 
the following results: 

1. You save time in measuring various 
amounts of conduit and wire required, since 
you have the location of the motor within 
5 ft. 

2. You can spot down any quantities of 
motors in a very short time. In fact, with 
practice it can be done in a space of time little 
more than just walking through the plant. 

3. You can take the sketch with you to 
your office or home and lay out your wiring 
plan free from many distractions or outside 
disturbances. 













10 


Profitable Power Wiring 


It has been deemed best to prepare a number of 
typical examples and by illustration show just how the 
installation should be laid out. It is not intended that 
these examples will cover the entire subject, because 
every motor job is different. However, the various 
examples shown cover a wide range, and it is hoped 
that they will serve their purpose. 

Example 1 

In this instance the customer is fortunate, for all of his 
motors are located in a space where the greatest distance 
of any motor from the service is only 20 ft. And all of 


SERVICE 




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i—i 

i— I 

8 * 

1-1 

1 l 

1 i 


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1-c 

8 

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1-1 

1-1 

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the motors are in a space 32 ft. x20 ft., where no indi¬ 
vidual run is over 20 ft. The proper thing to do is to 
mount our panel right at the entrance of the service. In 
this panel we incorporate our main cut-out and a separate 
and individual fuse gap for each motor. It is good prac¬ 
tice to provide one spare fuse gap for future additions. 
This fuse gap should be approximately 10 per cent of the 
total capacity, so that, in the future, should more motors 
be added this gap may be used as a motor circuit or a 
feeder circuit. The cost of this additional gap is compara¬ 
tively little, and it will most certainly prove beneficial. In 
this instance the panel board should be" mounted upon 
the wall where the service enters. 

Example 2 

In this instance we have the same number of motors, 
situated relatively the same, only they arc on the opposite 
side of the building from where the service enters. 

It is obviously out of the question to run separate and 
individual lines of conduit from the service to each of the 
motors, because that would be a great expense. Likewise, 
















The Design of Power Wiring Systems 11 

it wouldn’t be a good job to run a feeder over there and 
then mount cut-outs on the ceiling, tapped off from the 

SERVICE 



feed. No, the proper method would be to install one main 
cut-out at the service. Run from there a main feeder and 
terminate that feeder in a power panel mounted on column 
“A.” This power panel should have separate fused circuits 
for each of the motors and a spare gap equivalent in ca¬ 
pacity to 10 per cent of the total load. 

It might be asked, why not run the feeder to the wall 
and mount our panel board on the wall “B”? That method 
is perfectly correct, if the advantages of having the panel 
mounted on the wall are sufficient to warrant the extra 
cost of running additional feeder conduit cable and labor 
from “A” to “B.” Ordinarily the advantages gained 
would not be great enough to warrant the extra cost. 


Example 3 

This shows two separate groups of motors widely sepa¬ 
rated and situated at opposite ends of the building. 

To wire for this it is necessary to run two separate 
feeders from the service, one to each group. Therefore 
we provide a feeder panel at the service which will con¬ 
tain our main service fuses and two separate fuse gaps, 
one for each feeder, our main fuse gap to be heavy enough 
to carry the entire load, our two branch fuse gaps to be 
sufficiently heavy to carry the entire load of each group 
of motors plus about 10 per cent extra capacity. In a 
situation like this, it is frequently advisable to provide a 
spare gap in this panel for future feeder additions. Run 
each feeder to the groups of motors as shown and 













12 


Profitable Power Wiring 


mount the panel board at “A” and “B.” Each of these 
panels should have a separate fuse gap for each motor 



and one additional spare which is to be heavy enough for 
a load of about 10 per cent of the total horsepower fed by 
the panel. 


Example 4 

This is identical with the previous example except that 
an additional group, of motors has been installed at “C.” 
Now, had we provided that spare gap at the service as 
described in Example 3, all that would be required would 
be to run a separate feeder to “C” and to connect it to 



the spare gap provided in our main panel as mentioned 
before. At “C” install a distribution panel with separate 
fuse gaps for each motor and a spare fuse gap equal to 
about 10 per cent of the total amperes provided in the 
panel. 


Example 5 

In this motor layout our motors are all situated on the 
far side of the building away from the service. One 
method that is considered good practice would be to run 




























The Design of Power Wiring Systems 


13 


our feeder to “A." Install our distribution panel at “A/* 
providing separate gaps for each motor and a spare gap 
for future additions. It happens that there were only 


SERVICE 





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B 1 

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1 r ^ 

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A 


seven motors. If we had had a dozen or more motors,, 
as in the next example, we would proceed differently. 

Example 6' 

They would be divided into three separate groups. At 
“A” would be installed a combination feeder and distribu- 

SERVICE 



■ ■ 

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| - 


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BA C 


tion panel. This would have separate and individual gaps 
for the motors in group “A” and in addition two feeder gaps, 
for groups “B” and “C.” Feeders would be run to “B” 
and “C,” and a power panel installed at “B” and “C,” 
each to contain separate fuse gaps for each of the motors* 
with a spare gap for future additions. 





























14 


Profitable Power Wiring 


Example 7 

This example is identical with the previous one except 
that near the service there are a few motors. Therefore 


service 



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BA C 


we install a panel at the service that combines gaps for 
our main cut-out, our main feeder to “A” and separate and 
individual gaps for our motors at the service. 


Example 8 

If we have, in addition to the motors previously shown, 


SERVICE 


1- 1 

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B A. C 

a scattering of motors as shown above, a power panel may 
be put in at “D,” connected through a tap on our main 
feeder. 

Where our service enters in the extreme corner of 
a building, conditions are different and a few illustra¬ 
tions will serve to show how the work is handled. 





























The Design of Power Wiring Systems 15 

Example 9 

When motors are grouped near the service, we provide 






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1 1 

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1 1 

i m 

h—| 

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1- 1 

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SERVICE 


our main cut-out and distribution panel in one, providing 
a spare gap as previously explained. 

Example 10 

Here our motors are in a group a long way from the 
service. We therefore run a main feeder of proper size 



to this group and terminate it in a distribution panel. 
This should have a separate circuit for each motor and a 
spare gap equal to 10 per cent of the total horsepower con¬ 
nected. 


Example 11 

This is a combination of 9 and 10 and consists of a com¬ 
bination panel at the service containing a main fuse gap, 































16 Profitable Power Wiring 

a feeder gap, and separate gaps for each motor with one 






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SERVICE 

spare gap. The feeder is then run from the feeder fuse 
gap and is put in as shown in Example 9. 

Example 12 

Here we put in our combination panel containing main 
fuse gap, feeder gap, individual motor gaps with a spare. 
Run a feeder large enough for the remaining two groups 



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SERVICE 


and terminate at the second panel, which will contain a 
feeder gap and individual motor circuits with a spare. Run 
a feeder from the feeder gap to the last group of motors, 
terminating in a distribution panel with individual circuits 
and a spare. 


Example 13 

Install main cut-out at service and run main feeder to 

























The Design of Power Wiring Systems 


17 


first panel. This panel is to have separate and individual 



gaps and a feeder gap to next panel which will have sepa¬ 
rate and individual gaps and a spare gap. 


Example 14 


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1 1 

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L- 

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1 - 1 

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SERVICE 


This is the same as 13 except that a few separate and 
individual gaps are combined with the main fuse gap. 

So far we have been discussing a wiring layout for 
buildings of only one story. Where buildings have 
two or more stories, conditions are a bit different, but 
in reality not so very much. The simplest way to go 
about and lay out a wiring installation is as follows: 

Draw a plan of each floor and set them next to one 
another as shown below: On each floor plan spot the 
various motors in their respective locations. Now, by 
setting them next to one another, it is easy to see just 



























18 


Profitable Power Wiring 


how the motors are located with respect to one another 
on the various floors and from that it becomes easier 
to lay out the wiring system. 

Example 15 

In this case we would run along the cellar ceiling, where 
it is usually free and clear, our main riser to a point "A” 
where we install our first distribution panel. We select 
“A” for two reasons. One is that we can put our panel 
in the cellar to feed three motors at this point, and then 
we can run a riser up to “B” to take care of the motors 
on the first and second floors. 

Our panel at “A” will have three separate gaps for the 



SERVICE 

Cellar Main Floor First Floor Second Floor 


three motors, a spare gap and a feeder gap for the riser 
going up. This verticle riser will be of sufficient capacity 
to carry all of the motors on the upper floors and will 
terminate at a point “B” on the first floor. We can at 
this point put in a panel having enough circuits to provide 
for motors on the first and second floors, or we can put 
one in at “B” with circuits for four motors on that floor 
with a spare gap, and continue our feeder to the next 
floor and put in another panel for four motor circuits and 
a spare gap. For all practical purposes, an 8-circuit panel 
on the first floor with one spare gap will serve perfectly. 

Example 16 

Here we have a condition that is simple. We run a 
main feeder up to “A” on the third floor. Install a panel 












































The Design of Power Wiring Systems 


19 


with a feeder gap, three separate motor circuits for the 
three motors on the fourth floor and a spare. Run a sub- 



SERV1CE 

Cellar Main Floor First Floor Second Floor Third Floor Fourth Floor 


feeder to “B,” where we have a panel with nine circuits 
for nine motors; six on the third floor and three on the 
fourth floor and one spare. 

Example 17 

Here we run our main feeder to the third floor and 
mount a panel at “A.” This panel can be made to serve 
the motors on the second, third and fourth floors. This 



Cellar Main Floor First Floor Second Floor Third Floor Fourth Floor 


will contain circuits for a total of sixteen motors and one 
spare circuit. 

This will serve the purpose fully as well as three panels, 
one foreach floor, and will cost somewhat less money for 
material and labor. The amount of motor circuit run is 
the same. There is a saving in running 10 ft. of riser 
conduit and cable and only one panel is put up in place 
of three. 

The foregoing has served to illustrate the steps to 
take in laying out a wiring job. There has been no 
mention made of horsepower of the individual motors. 
This subject will be taken up in the next chapter. 



















































CHAPTER TWO 


The Design of Power Wiring Systems 

PART II 


I F the horsepower per motor ranged % hp. or less it: 

is not worth while to put the motors on separate- 
circuits. The procedure in that case would be to run 
a feeder and tap on with the individual motor switches 
as shown by the sketch. 

Where motors range from 2 to 25 hp. there are no 
especial precautions to be taken except in the case of 
alternating-current motors, and that is discussed in a 
separate chapter. 

Where motors are 35 hp. and larger, running up to 
100 hp., it is well to go at it very carefully. 

Where large motors are involved the layout should 
be carefully designed. For example: 

Assume that we have to wire up two 60-hp. 230-volt 
motors that are situated at a distance of 100 ft. run 
from the service. Should we run one feeder to the 
point where they are mounted, and should we then put 
in our distribution panel? Or should we mount our 
distribution panel in connection with our main cut-out 
and run two separate feeders? 

Assume there will be two elbows. Then we have 
the following combinations possible: 


7 




DISTRIBUTION 

PANEL 
























22 


Profitable Power Wiring 



PANEl 




^2 CIRCUITS IN 




one conduit 


56 

66 



y y 

J 

y y 

L 


ft 

ft 


DISTRIBUTION STARTING SWITCHES 

PANEL 

Amp. per motor equals 220. 

Amp. fdr two motors equals 440. 

Table A, N. E. Code, conductor per motor 220 amp. should 

be 4/0. 

Table A, N. E. Code, conductor for two motors 440 amp. 
should be 600,000 CM. 

Two 4/0 wires go in 2-in. conduit. 

Four 4/0 wires go in 2^-in. conduit. 

Two 600,000 CM. cables go in 3-in. conduit. 

As each of the three methods will require: 

1 Main cut-out 

1 Distribution panel with two sets of fuse gaps 

2 Switches 

we will leave them out of our calculation and figure 
out what our costs would be for each method, using 



For Very Small Motors Separate Circuits Are Necessary 




















































The Design of Power Wiring Systems 


nominal figures for prices of conduit and cable and for 
detailed labor costs. 


Method 1 

100 ft. 3-in. conduit at $0.45 per ft..$ 45.00 

2 3-in elbows at $3.00 each_$ 6.00 

200 ft. 600,000 CM. at $0.60 ft_$120.00 


Material _$171.00 

Labor Man Hours 
Pipe 38 
Elbows 38 
Wire 20 


60 Man Hours at $1.25 per hr—$ 75.00 


Total Cost ..$246.00 

Method 2 

200 ft. 2-in. conduit at $0.25 per ft.$ 50.00 

4 2-in. elbows at $1.00 each.$ 4.00 

400 ft. 4/0 at $0.22 per ft.$ 88.00 


Material .$142.00 

Labor Man Hours 
Pipe 48 
Elbows 3 
Wire 29 


80 Man Hours at $1.25 per hr....$100.00 


Total Cost.$242.00 

Method 3 

100 ft. 2^4-in. Conduit at $0.35 per ft.$ 35.00 

2 2}4-in. Elbows at $1.50 each.$ 3.00 

400 ft. 4/0 Wire at $0.22 per ft.$ 88.00 


Material .$126.00 

Labor Man Hours 
Pipe 32 
Elbows 2 
Wire 20 


54 Man Hours at $1.25 per hr....$ 67.50 


Total Cost 


.$193.50 




















24 


Profitable Power Wiring 


From an examination of the figures that have just 
been given, we arrive at a number of very interesting 
conclusions. 

Any one of the three methods is approved both 
from an engineering and from fire hazard standpoint. 
Any of these three methods will secure and maintain 
satisfactory operating service. 

The contractor who furnishes the wiring in accord¬ 
ance with method No. 3, is giving his customer just 
as good a job as the contractor who is planning on using 
No. 1, but the cost is 20 per cent less. 

Therefore, in conclusion of this particular phase of 
the matter it is well to bear in mind the following 
points: 

1. Do not be satisfied with just one scheme. 

2. Think out as many different layouts as pos¬ 
sible, and check the various costs, as in this way 
it may be perfectly practical and simple to cut 
your costs and still produce a good piece of work. 

Owing to the rules and regulations of the Fire In¬ 
surance Underwriters, each conductor is permitted to 
carry a certain number of amperes. 

The amount of current permissible does not vary 
directly in proportion to the amount of copper con¬ 
tained. For example: A No. 10 wire having an ap¬ 
proximate cross section of 10,000 c.m. is permitted to 
carry 25 amp. A No. 4 wire having a trifle over 
40,000 c.m. is permitted to carry 70 amp.; although 
the area is four times as great, the current is not 
three times as great. 

A No. 1 wire having an area of 80,000 c.m. is per¬ 
mitted to carry 100 amp. In other words, a No. 1 
conductor has eight times the amount of copper as a 
No. 10, and is only permitted to carry four times the 
amount of current. 


The Design of Power Wiring Systems 


25 


A 1,000,000 c.m. conductor, having 100 times the 
area of a No. 10 conductor is only permitted to carry 
twenty-two times the amount of current. 

So, therefore, we must take into consideration these 
factors and endeavor, in so far as it is possible, to 
use the most economical combination. This can be 
done by running conductors in multiple and by the 
use of additional distribution panels. 

By running conductors in multiple, we are able to 
take advantage of the higher carrying capacity per¬ 
mitted. This applies mainly for currents between 225 
and 450 amp., and allows us to use conductors 1/0, 
2/0, 3/0 and 4/0 which are standard, are carried in 
stock everywhere and are lowest in cost. Besides that, 
two No. 4/0 wires are easier to handle than one 
600,000 c.m. cable. Besides this, four No. 4/0 wires 
will go into one 2^4-in. conduit, whereas two 
600,000 c.m. cables require a 3-in. conduit, which cost 
more to buy and to handle. 

For direct-current work it is permitted to use one 
conductor per conduit. A 2-in. conduit will carry one 
1,000,000 c.m. cable, therefore for a load of 150 hp. 
we may use two 1,000,000 c.m. cables in two 2-in. 
conduits. 

In alternating-current systems all conductors must 
run in one conduit. Where it is a 3-phase system, 
all three wires must be in one conduit. Where it is a 
2-phase system, four wires must be run in one conduit. 
If the load is 300 hp., it may be divided into four 
circuits of 75 hp. each, three of 100 hp. each, or two 
of 150 hp. each, or perhaps two circuits of 100 hp. 
and 200 hp. each, but under any circumstances, all 
conductors of a circuit must be in one conduit. 





- • 



































































CHAPTER THREE 


Buying and Installing Panel Boards 


A PANEL, board usually consists of a slate or 
marble slab on which are assembled fuse gaps 
or switches or both, connected together by copper bus 
bars. This equipment is then mounted in an iron 
cabinet. The cabinet is usually larger by several inches 
than the slate or marble slab so as to provide a gutter 
or runway for the conductors entering and leaving 
the panel. 

Originally, before there were panel boards, when¬ 
ever wire sizes were changed it was necessary to install 
a fused cut-out, 2, 3, or 4-pole to protect the smaller 
size wire. These fused blocks of porcelain or slate 
were usually mounted together in a wooden box lined 
with asbestos or some such similar material and splices 
were made on to the main wires for each of the taps. 
In many instances there were as many as 50 fuse 
blocks in a cabinet, and every time a new circuit was 
added it was necessary to make another splice. 

Now, the making of a well-soldered splice is not a 
simple task and it takes time for labor and material, 
and a frequent application of the blow torch affects 
the quality of the rubber insulation on the conductors. 
The appearance of the work is unsatisfactory and the 
conductivity of the soldered joints is not always one 
hundred per cent. 

It is possible to use solderless connectors in place of 
soldered joints and where these connectors are of 



28 


Profitable Power Wiring 


proper size and type, a good piece of work is produced. 
But they must be put on carefully and they must be 
of the exact size to fit the wire, and when the first 
cost of these connectors is figured up, the average 
contractor of today will use panel boards that have 
been manufactured in accordance with his specifica¬ 
tions. 

Panel boards may be sub-divided into the following 
classes, applicable to motor work: Feeder distribu¬ 
tion panels; sub-feeder distribution panels; motor cir¬ 
cuit distribution panels. 

A panel board that takes the main service wires 
and has on it two or more fused switches or fused 
gaps controlling feeders is called a feeder panel. 
Where one of these feeders runs into a panel where 
the feeder is again divided into two or more sub-feed¬ 
ers it is called a sub-feeder panel. Where a service or 
feeder runs into a panel and is subdivided through 
fused gaps or fused switches into separate and indi¬ 
vidual motor circuits, it is called a motor circuit dis¬ 
tribution panel or more commonly known as a power 
panel. It is perfectly practical and possible to have 
all of these three types of panels combined into one. 

The question may arise as to whether it is better to 
have fused switches in the panels or just plain fused 
gaps. And whether it is better to use NEC fuses than 
open link fuses. We have thus four combinations 
available: 

1. Plain link fuses. 

2. NEC cartridge fuses. 

3. Link fused switches. 

4. NEC fused switches. 

By link fuses are meant fuses made up of a round 
fuse wire, or a ribbon fuse, or a piece of thin copper 


Buying and Installing Panel Boards 29 

cut down in the center, with clips at both ends to fit 
under the binding posts. 

In some localities it is against the rules to employ 
open link fuses without a fireproof inclosure, and in 
some localities they are not permitted at all. In com¬ 
munities where their use is allowed, there are usually 
exceptions, particularly in places where there is a 
continuous proximity of inflammable material, such 
as garages, flour mills, celluloid factories, etc. 

By NEC or National Electric Code fuses is meant 
a fuse that is inclosed in a tubular fiber casing, which 
prevents any spattering of molten metal when the fuse 
blows on short circuit. 

' All NEC fuses are refillable. That is, a burnt-out 
fuse may be sent to the manufacturer who, for a 
nominal charge, will replace the fuse element. 

There are also on the market, however, a number of 
types of NEC renewable fuses which differ from the 
ordinary NEC type in that the consumer is able him¬ 
self to replace the burnt-out fuse element. 

It has been found that on sizes 400 amp. and over 
that the NEC type of cartridge fuse generates heat 
and sometimes to a degree that is liable to cause 
trouble, and for that one reason alone it is usually ad¬ 
visable to use link fuses for circuits carrying over 400 
amp. 

The best kind of a job will call for NEC fused 
switches in all panels, controlling all circuits. Thus if 
it is desired to work on any circuit, all that is neces¬ 
sary is to open that switch and proceed to make re¬ 
pairs. That is the only object and purpose of installing 
switches in power panel boards. 

Actually, in practice, the need for making such re¬ 
pairs is not frequent and, when the work is properly 
laid out, occurs but seldom. Therefore, for all prac¬ 
tical purposes, a power panel that contains merely 


30 


Profitable Power Wiring 


NEC fused gaps is a perfectly proper and satisfactory 
job. For when some repair work is found necessary, 
an electrician is usually called in, and for him it is a 
comparatively safe and simple job to remove and re¬ 
place cartridge fuses. 


Open Link Fuses 

Now, then, it might be asked, and quite properly 
too, why not use the open link fuses since they are 
approved ? It is not satisfactory to use open link fuses, 
because, in the first place it is difficult to replace them, 
since all the parts are live and the danger of a short 
circuit is ever present. Secondly, it is so easy to re¬ 
place them with copper wire, which sooner or later 
is the cause of motor trouble. 

If link fuses are desired, it is always preferable to 
use link fused switches, where the switch may be 
opened and the circuit made dead; or else put in a main 
switch before the circuits, that may be opened so as 
to make it a safe and easy procedure to replace fuses. 
However, this method would require a complete plant 
shut down to replace one set of fuses, and therefore 
it is not used in the better class of work. 

There is a place, however, for link fuses and that is 
on the main feeder or power circuits where the am¬ 
perage runs over 400 amp. These main circuits, if 
the work is properly designed, seldom, if ever, blow 
out and the use of copper link fuses is satisfactory 
and approved. Such fuses are much cheaper than 
equal size cartridge fuses and the panels themselves 
are smaller and less costly. 

After it has been decided what motors will be used 
and the number and types of panels to be installed, 
the contractor calls up the switchboard manufacturer, 
quotes his requirements and receives an estimate. 


Buying and Installing Panel Boards 


31 


The manufacturer should be given the following 
information: 

Type of panel board, whether surface or flush. The 
first is mounted on the finished surface and is usually 
used when wiring is installed in finished buildings, and 
the flush type is set into the wall and the surface is 
flush with the wall surface, usually installed in new 
construction work. 

Type of metal cabinet, doors and hinges and types 
of catch. 

Kind of service, whether direct or alternating cur¬ 
rent. If direct, whether 110, 220, or 550 volts. If 
alternating current, whether 110, 220, 440, or 550 volts, 
single, two or three phase, 60 cycles, whether 2 wire, 
3 wire or 4 wire. 

Number of circuits and amperes per circuit and ar¬ 
rangement of circuits. 

Link fused or NEC fused. 

Knife switches, link or NEC fused. 

Size of conductors, entering panel. 

Approximate full load amperes on bus bars. 

Size pipe, knockouts on cabinet and their locations. 

Conductor Entrance Direction 

There is one point of great practical importance that 
has not been mentioned. That is—the size of main 
conductors entering the panel and the direction from 
which they come in. It is to the contractor’s interest 
to consider these points very carefully, for he will not 
only save money on his labor, but he will turn out a 
better job. 

In the first place, it is necessary to know from what 
direction the conductors are entering, that the panel 
ma\ T be designed, so that the main bus bar lugs will be 
right at that point, which makes it unnecessary- to carry" 


32 


Profitable Power Wiring 


these heavy cables all around the gutter and then a 
sharp bend into the lugs. 

Next, if it is known on what side these cables enter, 
we can provide a gutter that may be from 6 to 10 or 
even 12 in. in width and will permit us to sweat our 
cable into the main lug and then make it easy for us 
to bolt the lug into place without any severe strain on 
conductor or lug, on insulation or on the working¬ 
man’s temper. 

Most of the fires that start in feeders in conduit are 
caused by defective insulation damaged by attempting 
to manipulate and bend heavy conductors into sharp 
bends and into spaces that are much too small. 

As a word of caution, it is recommended to provide 
plenty of space at the point where your conductors 
enter the panel cabinet and to have the main lugs as 
near in line as practicable. Any small extra expense 
in making a larger steel cabinet is more than offset by 
the saving in labor and a better job. 

Panel Layout 

Therefore, it is always advisable to leave the layout 
of the panel to the manufacturer, who through ex¬ 
perience and knowledge can usually suggest ways and 
means whereby the panel may be turned out at the 
lowest price, consistent with high quality. If a job is 
carefully planned and laid out it is very seldom neces¬ 
sary to replace fuses in any of the power panels. 

For panels containing four or less circuits, no gutter 
is required. For more than four circuits it becomes 
necessary to provide a gutter unless circuits are brought 
out directly at the point where they leave the fuses. 
Ordinarily it would cost more to bring conduit elbows 
and condulets to the side of the panels than it would 
cost to provide a gutter; therefore it is recommended, 
from a practical point of view, always to provide any 
panel containing more than four circuits with a gutter. 


Buying and Installing Panel Boards 


33 


Where a job is Small and it is desired to do good 
work at a low cost, the following scheme has been 
tried and found satisfactory. It consists in using ordi¬ 
nary fused cut-outs which are assembled in such fash¬ 
ion as to do away with the necessity of making any 
special joints. 

Assume, for example, that we need a panel for six 
7^-hp. d.c. motors. Our feed, coming in, consists of 
two No. 4/0 wires, and our individual motor circuits 
will require No. 8 conductors. We will need six 2 
pole NEC fuse gaps 30-60 amp. Cook in your catalog 
for the dimensions of a 2 pole main line 30-60 amp. 
NEC fuse cut-out. This shows its overall length 
parallel to the fuses is 5 in. and its width across is 
35/s in. 

In this instance we have to use six of them. Their 
length will be 5 in. and the width of each 35/s in. 
The width ot six of them will then be, roughly, 22 in. 
Allowing 2 in. on either side will make our panel 2 
plus 5 plus 2, or 9 in. wide. Allowing \/ 2 in. between 
each cut-out will give us 22 plus 7/ 2 in., or a height 
of 29/ 2 in., or about 30 in. Our wires will come out 
at all of the sides so a gutter of 3 in. will be ample, 
except on the side where our main wires are coming 
in, and as they are 4/0, plenty of space should be al¬ 
lowed, and a gutter of 6 in. at least should be pro¬ 
vided. Our cabinet will then be 3 plus 9 plus 3, or 
15 in. wide, and will be 3 plus 30 plus 6, or 39 in. 
high. Our door will be 9 by 30 in. and will be set a 
little off center. 



Layout of a Six Circuit Panel 





















34 


Profitable Power Wiring 


Layout of a Six Circuit Panel 

We now mount our cutouts in the box, allowing 1J4 
in. between the edges of each. Our box is made so 
that there are standard p 2 -in. knockouts in the sides 
of the gutter. 

Connect to one end of the cutouts wires of one 
polarity and bring them around to the bottom. Skin 
the ends and solder all of them into one 4/0 lug. 
Solder a 4/0 lug on the end of the main conductor and 
bolt them together and tape well. Do the same now 
with the six wires of the other polarity, and you have 
a job that is, if properly done, both mechanically and 
electrically a satisfactory one, at a lower cost than 
panels, and whose appearance is quite workmanlike. 

A few words on the mounting and location of 
power and feeder panels might be well added. Panels 
such as we have been describing consist of fuses for 
the protection of motors and wiring and if properly de¬ 
signed require little attention except the occasional re¬ 
newal of a fuse. 

They should have numbers on the various circuits to 
identify them, and a blue print should be mounted in¬ 
side the panels to correspond and to show what motor 
or what portion of the power wiring each individual 
circuit controls, so that in case of failure to burn out 
it will not be necessary to test out each circuit to find 
out where the trouble exists. For in cases like that, 
speed is a vital factor. It is a good idea to have a 
little box mounted on the door to hold one or two 
spare fuses, so that no time will be lost in replacing 
them. 

For this reason, namely, speed, it is essential that 
panels be easily accessible. They should not be mount¬ 
ed on the ceiling where it requires a ladder to reach 
them. They should be securely mounted against a 
side wall, at such a height that a man standing on the 
floor can reach any fuse without difficulty. Sometimes 


Buying and Installing Panel Boards 35 

conditions are not ideal, and it is necessary to mount a 
panel on a column, and sometimes near the ceiling. 
Care should be taken that materials, boxes, lumber and 
crates and other things are not stacked in front of 
them, making it impossible to reach them and some¬ 
times making it impossible to open the doors. Where 
space is confined and small it is well to split the door 
and make two small ones instead of one large one. 
Then the door should be hinged on the long side so 
that it requires the smallest space for opening it. 


























* 
























































* 













- 













• ' 












CHAPTER FOUR 


How to Figure D. C* Power Wiring 


I N' laying out any power job for estimating it is very 
easy to jump at what seem obvious conclusions, 
and in that way arrive at a cost that will not give a bit 
better job than one done a little differently but also a 
little less expensively. This more economical way can 
be discovered only by working out each job. It may 
take more time, but the results will be more certain. 

.In this chapter the subject of direct-current motor¬ 
wiring estimating is taken up in a way- calculated to 
show the value of working out each problem step by 
step and also how to do the figuring. Examples are 
worked out from actual installations, bringing in as 
many different conditions as will probably ever arise. 
The same plan will be followed later for alternating- 
current work. 


Full Load Amperes of d. c. Motors 


Service 


Horsepower of Motor 


Voltage 

X 

K 

X 

1 

IK 

2 

3 

5 

7K 

10 

15 

20 

25 

30 

40 

115 

2.4 

4.8 

7.0 

9.2 

13.2 

17.2 

25.0 

40 

60 

78 

116 

152 

188 

224 

294 

230 

1.2 

2.4 

3.5 

4.6 

6.6 

8.6 

12.6 

20 

30 

39 

58 

76 

94 

112 

147 

550 



1.5 

1.9 

2.8 

3.6 

5.3 

8.4 

12.5 

16.3 

24 

32 

39 

47 

62 




























38 


Profitable Power Wiring 


Example 1.—Service at 115 volts. 

For our first example let us take the layout shown 
here where all of the motors are clustered about the 



i m 

i i 

tt'tt-W'Sli 





V V 

1 V 















service and where no feeder is necessary. Service is 
115 volts direct current. 

We have total of seven motors as follows: 


Motor No. ’ 

Horsepower 

Amps, per Motor 

1 

15 

116 

2 

10 

78 

3 

7H 

60 

4 

5 

40 

5 

3 

25 

6 

2 

17.2 

7 

1 

9.2 

7 

43H 

345.4 


From these figures it would appear that motors at 
115 volts direct current draw approximately 8 amp. 
per horsepower. 

If in this instance service is taken from a local light¬ 
ing company and the contractor is required to provide 
the wiring necessary to bring it inside the building, we 
proceed as follows: 

Add to the total of 345.4 amp. 10 per cent additional 
for the future, or 34.5 amp. to secure the total amper¬ 
age required, namely, 379.9—or 380 amp. 

There will be two conductors in one iron conduit, 
for generally the central station rules do not permit 
multiplying of conductors or conduits. 
















How to Figure D. C. Power Wiring 


39 


Referring to the wiring tables of the National Elec¬ 
tric Code, we find that 380 amp. permits the use of 
500,000 CM rubber covered or 400,000 CM varnished 
cloth-covered wire. 

For conduit sizes, two conductors in one conduit, the 
code requires 3-in. conduit for both the above wires. 
Eabor costs will therefore be very nearly the same, and 
the question of choosing between 500,000 CM rubber 
covered and 400,000 CM varnished cambric becomes 
one of price and delivery. 

Having decided on the size of conduit and wire to 
use, we next consider the main service switch. To be 
of sufficient capacity to carry 380 amperes it will be a 
2-pole, 400-amp. single-throw, link or cartridge fuse. 
This may be of the plain type mounted in a steel 
cabinet or of the safety type with the control handle 
outside. 

We then allow and make provisions for the meter, 
meter panel and test blocks, and run into our main 
distribution panel. This panel will contain a main 
cutout, seven separate and individual fuse gaps for 
the seven motors and one spare circuit. 

To proceed with this work it is recommended that 
a table be prepared as follows: 


Panel 

Board 

Circuit 

No. 

Motor 

HP 

Motor 

Amps. 

Size Gap 
or 

Switch 

Amps. 

Size 

Fuse 

Size 

Con¬ 

duc¬ 

tors 

Size 

Con¬ 

duit 

Size 

Motor 

Switch 

Fused 

Size 

Fuses 

Size 

Motor 

Switch 

Unfused 

1 

Main 


400 

400 






2 

15 

116 

200 

150 

2-0 

m 

200 

125 

200 

3 

10 

78 

100 

100 

2 

IK 

100 

90 

100 

4 

7K 

60 

100 

70 

4 

IK 

100 

65 

60 

5 

5 

40 

60 

60 

6 

1 

60 

50 

60 

6 

3 

25 

60 

35 

8 

K 

60 

35 

30 

7 

2 

17.2 

30 

25 

10 

H 

30 

25 

30 

8 

1 

9.2 

30 

15 

14 

K 

30 

15 

30 

9 

Spare 


60 


































40 


Profitable Power Wiring 


We first will take up the ordering of our distribu¬ 
tion panel. This will be designed for direct current, 
115 volts, and will contain one main fused circuit and 
eight separate branch circuits for seven motors and 
one spare. It may be provided with a main switch 
and with separate branch switches. The underwriters' 
rules do not require them, and it remains a matter of 
choice. In work of the highest quality where cost is 
a secondary matter, switches are usually furnished. 

Lugs on the main bus bar should be great enough to 
each take 500,000 CM conductors and the main bus 
bars should be of such size that they will carry a total 
of 400 amperes. The separate branch circuits should 
be of the capacity shown in the above table. 

Main fuses may be link type or cartridge type. From 
the standpoint of economy of space and material, link 
fuses are cheaper, although cartridge fuses are easier 
to replace. Branch fuses should preferably be of the 
cartridge type. 

In the design of this panel the main conductors will 
enter from the left. Therefore, arrangements should 
be made for a gutter of at least 6 in. wide on the left 
side. The main fused circuit should be so arranged 
that the main conductors will run right into the main 
lugs without twisting or bending. Gutters for the 
other sides should be at least 4 in. wide throughout. 
Knockouts for the entrance of conductors and for the 
exit of branch circuits should then be clearly specified 
and located. 

Branch circuits are run in accordance with the 
values shown in the above table. All motor circuit 
wires should be selected, by adding to the full load 
amperes of the machine a sum equal to 10 per cent of 
this amount and the conductor then picked out from 
Table A of the code for rubber-covered insulation to 
be used in conduit systems, and Table C for open cir¬ 
cuit work. Varnished cloth insulated wire may be 


41 


How to Figure D. C. Power Wiring 

used in conduit systems, but for sizes less than 350,000 
CM, the greater cost of the wire and the long deliveries 
more than offset the gain secured by the use of smaller 
conduits and conductors. 

Since each circuit is fused at the distribution panel 
it is not necessary to furnish a fused switch. In this 
way it is possible sometimes to use a smaller size 
switch. This is shown in the case of the 7^-hp. motor 
which has a full load ampere rating of 600 amperes. 
It would be poor practice -to put in a 60-ampere fused, 
while a 60-ampere unfused switch would be perfectly 
correct. Where fused switches are used, it should be 
made a practise to install smaller fuses at the switch 
than in the panel so that in case of burn-out it would 
save time to investigate trouble. 

Example 2.—A 230-volt Installation. 

The next type of installation to be worked out will 

SERVICE 


% E 

|| 

1 | 

1 ; | 





-1 

|| 

■ a 

i 

i 

■L m 







■ ■ 



1 1 

l I 


be a 230-volt direct-current system where there are 
seven motors of the following ratings: 


Motor No. 

Horsepower 

Amps. * 

1 

25 

94 

2 

15 

58 

3 

10 

39 

4 

5 

20. 


3 

12.6 

6 

2 

8.6 

7 

1 

4.6 




Totals 7 

61 

236.8 























42 


Profitable Power Wiring 


Nearly 90 per cent of the direct-current motor in¬ 
stallations are 230-volt. For ordinary wiring purposes 
the total amperage at 230 volts d.c., as is apparent 
from the table, may be obtained by multiplying total 
horsepower by four. 

To the total amperage or 236.8 add 10 per cent to 
allow for future additional horsepower, and the result 
is 260.48 amp. Referring to our table for conductor 
and conduit sizes we find: 

Rubber covered—2 No. 300,000 CM in 2]/ 2 -in. con¬ 
duit. 

Varnished cloth—2 No. 4/0 in 2-in. conduit. 

It is apparent, then, that it is more economical in 
materials and labor to use varnished cambric conduc¬ 
tors, No. 4/0, in 2-in conduit. This conduit will 
terminate in a cabinet containing one main 2-pole, 
single-throw fused service switch. To carry 260.5 
amp. it will have to be of 400-amp. capacity and NEC 
cartridge fuses are recommended. 

From thence we leave facilities for the connection 
and location of the meter and continue to the house 
side. An examination of the sketch shows a long 
feeder following the main cut-out which, in this in¬ 
stance, should be a 2-pole or 2-single pole 400-amp. 
NEC fused cut-outs. The feeder from there must 
carry 260.5 amp., so we may use any one of the fol¬ 
lowing combinations: 

Two rubber-covered 300,000 CM conductors in one 
2^-in. conduit. 

Two rubber-covered 300,000 CM conductors in two 
l^-in. conduits. 

Two varnished cambric 4/0 conductors in one 2-in. 
conduit. 

Two varnished cambric 4/0 conductors in two 1J4- 
in. conduits. 

Four rubber-covered 2/0 conductors in one 2j^-in. 
conduit. 


How to Figure D. C. Power Wiring 43 

Four rubber-covered 2/0 conductors in two 2-in. 
conduits. 

Prices for materials and labor per foot for each of 
these combinations show the lowest to be two No. 4/0 
varnished cambric in one 2-in. conduit. 

Some contractors might say, why spend this time 
in making up various combinations? And the reason 
is that the ordinary contractor will use two No. 300,000 
CM conductors in 2j^-in. conduits where his materials 
alone cost him 20 per cent more per foot and his labor 
at least 25 per cent more. The man who sits down 
to work out these details can either put in the same 
price and make more money, or else can take it at a 
lower figure and still make as much as his competitor. 

All the time, one job is just as good as the other. 
Although more copper is used by putting in 300,000 
CM R. C. cable it is not permitted to carry more cur¬ 
rent than the 4/0 varnished cambric cable, and there¬ 
fore, as far as the customer is concerned, his installa¬ 
tion is every bit as good. 

Our feeder of two No. 4/0 V.C. cables in one 2-in. 
conduit will then terminate in a distribution panel to 
contain eight circuits for seven motors and one spare. 


Panel 

Circuit 

Motor 

HP 

Motor 

Amps. 

She 
Switch 
or Gap 

Size 

Fuse 

Size 

Con; 

ductor 

Size 

Con¬ 

duit 

Size 

Motor Switch 

Size 
Fuse 
_/ 

Fused 

Unfused 

1 

25 

94 

200 

125 

1-0 

PA 

200 

200 

110 

2 

15 

58 

100 

75 

4- 

PA 

100 

100 

70 

3 

10 

39 

60 

60 

6 

1 

60 

60 

50 

4 

5 

20 

80 

30 

10 

H 

30 

30 

25 

5 

3 

12.6 

30 

20 

12 

% 

30 

30 

15 

6 

2 

8.6 

30 

15 

11 

H 

30 

30 

10 

7 

1 

4.0 

30 

15 

14 

H 

30 

30 

10 

g 

Spare 


30 

| 














Total.. 

61 

236.8 









From the data above panel board specifications are 
prepared. Just branch fuse gaps or branch fused 























44 


Profitable Power Wiring 


switches may be specified. The main bus bars should 
be designed to carry a total load of 236 plus 10 per 
cent, or 259.6 amp. The main terminal lugs should be 
large enough for No. 4/0 conductors, and as the feeder 
will enter at the top of the board, lugs should also be 
at the top. The top gutter should be at least 5 in. in 
width and the other gutters on the remaining three 
sides should be at least 4 in. wide. From the panel we 
then run separate circuits in conduits and wire as 
shown in the above table. 

Example 3.—Two Groups, 550-volts. 

Group A has five motors, 10, 7j^, 5, 3 and 2 hp., a 
total of 27^2 hp. Group B has four motors, 15, 1, 5 
and 2 hp., a total of 23 hp. The combined total equals 
50^ hp. From the table the current ratings of each 
motor are secured. Added together, they give a total 
of 84 amp. This is roughly 1 2/3 amp. per horse¬ 
power. 

Now work can be started on the service layout. All 
switches, all fuse cut-outs, fuses and panel boards 


SERVICE 


li i 

a n 

■ f 


« « 




m 

p§, 

|Sq 

1 is 

0 1 

■ m 

9 t 

i i 

1-1 

3 C 

, . 

"i * 

J t 

d 

PlilH 

•-1 

1-1 

;-1 

r -■ 

)- r 

) 1 



must be designed and built for 550 volts, which means 
greater spacing between bare conductors and special 
fuses. 

Service wires must carry 84 amps., and 10 per cent 
additional or 92.4 amps. 

The conductors will accordingly be two No. 1 R. C. 
D. B. stranded. Conduit will be \y 2 in. and the main 

















How to Figure D. C. Power Wiring 


45 


service switch will be 100 amps., 550-volt, 2-pole, 
single-throw fused, using NEC fuses. 

Provisions are then made for metering devices and 
fittings. 

According to the sketch there will be a main 
distribution cabinet containing the main cut-out and 
two separate feeders, one running to Group A motors, 
the other running to Group B motors. The first con¬ 
sideration, then, is to design the main feeder panel 
which requires, first of all, a determination of feeder 
sizes. 

Group A, with 27 hp. at 1 2/3 amps, per horse¬ 
power, would draw 45.6 amps, which, with the 10 per 
cent extra capacity would require a feeder to take 
50.16 amps. In like manner Group B would require 
a feeder to take 42.02 amps. 

According to these figures, each feeder may be in¬ 
stalled using two No. 6 in 1-in. conduit. While the 
figure 50.16 amps, is 0.16 amp. above code rating, the 
fact that 10 per cent supply capacity has been added 
voluntarily permits the use of this size wire without 
any hesitancy. 

The two feeder circuits then can be protected by 
two 60 amp. gaps while the main gap will be 100 amp. 
It is good practice to provide a spare gap and in this 
instance another 60 amp. gap would be proper. Since 
this service is 550 volts, which is more hazardous to 


Feeders 









































46 


Profitable Power Wiring 


handle than either 115 or 230 volts, it is recommended 
that all circuits be provided with switches in order 
that the danger involved in replacing fuses be reduced 
to a minimum. In fact, it is considered best practice 
to provide a panel equipped with safety switches 
throughout. The main panel will then consist of one 
main 100 amp. switch and one 60 amp. spare circuit 
switch, both 2-pole, single-throw 5 50-volt fused. 

If it is not feasible or possible to obtain such a panel 
promptly, it is possible to make up a job with standard 
safety switches, as shown in the sketch, particularly 
since there are only four switches altogether. 

The feeders are then run to separate panel boards 
at A and B. Panel board A will have six circuits for 
five motors and one spare which will be large enough 
for about 10 per cent of the total load controlled— 
namely, 3 hp., but as the smallest gap is 30 amps, the 
circuit will carry more if necessary. 


Circuit 

No. 

HP 

Amps. 

Size 
Gap or 
Switch 

Size. 

Fuse 

Size 

Con¬ 

ductor 

Size 

Conduit 
for 2 

Size 

Motor 

Switch 

Size 

Motor 

Fuses 

1 

10 

16.3 

30 

25 

No. 10 

H 

30 ' 

20 

2 

7 H 

12.5 

30 

20 

No. 14 

Vi 

30 

15 

3 

5 

8.4 

30 

15 

No. 14 

Vi 

30 

10 

4 

3 

5.3 

30 

15 

No. 14 

Vi 

30 

10 

5 

2 

3.6 

30 

15 

No. 14 

M 

30 

6 

6 

Spare 


30 















Total.. 

46.1 








This panel board, which will be designed for a 
2-wire 550-volt direct current system, may be equipped 
with or without switches, although switches are pref¬ 
erable. The main bus bars will have to carry 46.1 amp. 
and with the spare it would be good practice to call 
for buses having a carrying capacity of 60 amp. Tugs, 
on the main bus bars will have to be large enough for 
No. 6 conductors. These will be at the top or bottom, 
depending upon which side the feeder enters. Tugs 

















How to Figure D. C. Power Wiring 


47 


for each of the circuit wires are to be large enough for 
No. 8 conductors. A gutter 3 in. to 4 in. wide will be 
required, and no more. 

The Group B panel board will have 5 circuits for 
four motors and one spare. 


Circuit 

No. 

Hr 

Amps. 

Size Fuse 
Gap or 
Switch 

Size 

Fuse 

Size 

Con¬ 

ductors 

Size 

Con¬ 

duit 

Size 

Motor 

Switches 

Size 

Motor 

Fuses 

1 

15 

24 

60 

40 

No. 8 

V\ 

60 

35 

2 

5 

84 

30 

15 

No. 14 


30 

15 

3 

2 

3.6 

30 

15 

No. 14 

Vi 

30 

6 

4 

1 

1.9 

30 

15 

No. 14 

Vt 

30 

6 

5 

Spare 


30 














Total.... 


37.9 








As before, it may or may not have switches, par¬ 
ticularly since the feeder controlling this panel board 
can be disconnected through its switch. Bus bars 
should be designed for 60 amp. Terminal lugs, either 
top or bottom, should be large enough for No. 6 con¬ 
ductors. Circuit lugs should be large enough for No. 8 
conductors. 

An inspection of both tables shows the use of fused 
motor switches. It is necessary to use a motor switch 
but it is not necessary to use a fused motor switch, 
since the fuse in the panel board is sufficient to protect 
the motor circuit. However, if a fused switch is used, 
the fuses installed therein should be smaller than the 
ones installed in the panel board. 

Example 4.—Large Number, Small Motors. 

Very frequently the contractor will run across a 
piece of work which involves a large number of small 
motors, as, for example, the installation of thirty-two 
%-hp. 115-volt motors. 

Very frequently these 115-volt motors will be in¬ 
stalled in a plant where the service is 2-wire, 115 volts, 
or in a plant where the service is 3-wire, 115-230 volts; 

















48 


Profitable Power Wiring 


or, as in some cases, where the plant service is 2-wire, 
115 volts, and where it is desired at times to throw 
over to outside central station power where the system 
is 3-wire, 115-230 volts. 

We shall first consider a 2-wire 115-volt service. 

There are thirty-two %-hp. motors which take 2.4 
amps, per motor or 76.8 amps, to which should be 
added an extra 10 per cent, bringing the service re¬ 
quirements up to 84.5 amps. 

This condition can be met by using two No. 2 D.B. 
R.C. wires in one 1 ^-in. conduit. The service switch 
will be 2-pole, 100 amps., single throw, fused. Coming 
through our meter provision is made for a main cut¬ 
out, which should be 2-pole, 100 amp., NEC fused. 

The main feeder if carried straight over will come 

Service 


-iH.P 
115-Volt 
Motors 


about in the center of this line of motors. There are 
then several ways in which these motors may be wired. 

The first way would be to put in a double branch 
cut-out, run two sub-feeders right and left each to 
carry 42 amps., and to consist of two No. 6 wires in 
1-in. conduit, and on each side tap sixteen motors di¬ 
rect to the fused starting switch. 

The second method would be to split the thirty-two 
motors into four groups instead of two, running two 
sub-feeders to the left and two sub-feeders to the 
right, each to carry eight motors or 19.2 amps, and 



















How to Figure D. C. Power Wiring 


49 


consisting of two No. 10 wires in J^-in. conduit. The 
panel board would then contain four circuits instead 
of two, and these would be 30 amp. circuits instead of 
60 amp. as in the first method. 

While both of the two methods described are elec¬ 
trically’ correct, the second is the more practical and 
satisfactory for the following reasons: 

1. The conduit may be run right into the switch 
boxes. 

2. Standard boxes and conduit fittings may be 
used. 

3. No. 10 wire may be conveniently connected to 30 
amp. switches. 

All of these operations are easier to carry out if 
No. 10 wire and j4-in. conduit is used rather than No. 
6 wires and 1-in. conduit. 

The panel board will be designed for 115 volts, 
2-wire direct current, and will contain four separate 
30 amp. fused gaps or switches. Teads from the fused 
motor switches to the motors will be No. 14 wire, for 
although the current is only 24 amps, per motor, No. 
14 wire is the smallest permissible to run. 

Emergency Service at 115-230 Volts 

Now let us assume we have thirty %-hp. 115-volt 
motors to connect on a 115-volt direct current system 


Feeder for 8-3 H.P Motors 
Feeder for 7 3 H.R Motors 


Panel 



Feeder for 8*3 H.R Motors 
Feeder for 7-3 H.R Motors 


Main. Feeder 


f< 


•Doable-throw Switch 

i 


Plant 


T 


-230 Volts 


















50 


Profitable Power Wiring 


where it is desired to throw over to a 3-wire 115-230 
volt system, and where the requirements are such that 
the load must be equally balanced on each leg of the 
system. 

The simplest way to show this would be as follows: 

The only difference in wiring will be in the main 
feeder and in the power panel. The local distribution 
from the panel board to the motors will remain at 115 
volts, whereas the feeder system and the panel board 
will be of the convertible type—that is, either 115 
volts, 2-wire, or 115-230 volts, 3-wire. 

If the switch is thrown to the left across plant serv¬ 
ice, the two outer legs will be of the same polarity, and 
the difference of potential will be zero, while the dif¬ 
ference in voltage between the center leg and each of 
the outer legs will be 115 volts. 

If the switch is thrown to the right across the 
3-wire service, the difference in potential across the 
outside conductors is 230 volts and the difference in 
potential from the center leg to each of the two out¬ 
sides is 115 volts. So that in both services the voltage 
between the center conductor and each of the outsides 
is the same—namely, 115 volts. 

Consideration should now be given to the relative 
sizes of these feeder conductors. 

For plant service •—Total current at 115 volts is car¬ 
ried by the center conductor and half of total current 
at 115 volts is carried by each of the outside conduc¬ 
tors. Therefore, in determining the size of conduc¬ 
tors, care must be taken that the center leg is of suf¬ 
ficient capacity to carry the entire amperage at 115 
volts, and the two outside legs must each have suf¬ 
ficient capacity to carry half the load carried by the 
center leg. 

For Edison Service —Each of the outside wires will 
carry the total amperes at 230 volts and the center leg 
will carry only the unbalanced load. If the fuse on 


How to Figure D. C. Power Wiring 


51 


one side of a 3-wire 115-230 volt system goes out, the 
current in the center or neutral leg will carry the 
same current as that carried by the outsides. In this 
particular instance the current in the outside legs is 
identically the same for the thirty J4~hp. 115 volt mo¬ 
tors whether it is operating on a split 2-wire, 115-volt 
system, or a 115-230 volt Edison system. The center 
conductor will carry no current with a perfectly bal¬ 
anced load on the Edison 3-wire, 115-230 volt system, 
and will carry twice the current of each of the outside 
wires on a 115-volt split 2-wire system. 

The total current at 115 volts, direct current, for 
thirty %-hp. motors is 30 x 2.4 or 72 amps., which, 
with the 10 per cent extra, equals 79.2 or 80 amps. 

The center leg carrying 80 amps, takes a No. 2 con¬ 
ductor, while the outside legs carrying fifty per cent 
each, or 40 amps, take a No. 6 conductor. The conduit 
for these three conductors (one No. 2 and two No. 6), 
is 1% in- The double throw switch will be 100 amp., 
3-pole. 

Frequently, a 2-wire, 115-volt, direct current system 
has to be adapted to 3-wire 115-230 volts, so that it 
may be operated on either one or the other form of 
service. In principle it is a simple affair, but from a 
practical standpoint it involves a careful examination 
of the wiring system to determine what is to be done. 
Where any of the motors involved are not larger than 
10 hp., the Edison companies will permit such a change, 
provided the load is equally balanced. 

In some instances, it is desired to throw over from 
plant service to Edison service for breakdown purposes 
only. In such cases it is necessary to use a main double 
throw or separate and individual double throw switches. 

Where it is the intention to discard the plant and 
operate exclusively from Edison service it is unneces¬ 
sary to use any double throw switches. 


52 


Profitable Power Wiring 


Permanent Change From 2-Wire, 115-Volt to 3-Wire, 115- 
230-Volt. 

Case No. I. 

Here is an eight-circuit, 2-wire panel. Bring up an 
additional conductor A. Cut the bus bar at B, so that 
under the new system N is maintained as the neutral 
conductor, and the positive and negative each carry 
four circuits. The new negative conductor should be of 


On Plant £-Wlre 
115-Volt Service 


On Edison 3“Wire 
115-230 Volt Service 




such size as to carry one-half the total amperes form¬ 
erly registered at 115 volts and 2-wire. 

Case No. II. 

In this case there are two distribution panels, each 
carrying the same horsepower. Simply disconnect one 
of the main leads and replace it with another conduc¬ 
tor run from the service as shown. 
























How to Figure D. C. Power Wiring 


53 


a-Wi re. 115 Volta 3-Wlr«. 115*230 Volts 



Where there are a series of panels all fed from one 
main 2-wire feeder, add up the horsepower of all the 
panels, divide the load as near as possible: 


Cellar panel_ 10 hp. 

First floor panel ... 20 hp. 

Second floor panel.. 22j4 hp. 

Third floor panel. 40 hp 

Fourth floor panel... 16 hp. 

Fifth floor panel . 8 hp. 


Total ...116^4 hp. 













































54 


Profitable Power Wiring 


Combination possible on 3-wire system. 

One Side. Other Side. 

Cellar _ 10 hp. Third floor _ 40 hp. 

First floor _ 20 hp. Fourth floor_ 16 hp. 

Second floor _ 22*4 hp. 

Fifth floor _ 8 hp. 

60^4 hp. 56 hp. 

That would be considered fairly close, although a 
lower cost of wiring could be obtained by: 

Cellar ..10 hp. Third floor _ 40 hp. 

First floor _ 20 hp. Fourth floor _ 16 hp. 

Second floor . 22}4 hp. Fifth floor _ 8 hp. 


52hp. 64 hp. 

But the balance is not sufficiently good to warrant 
taking any chances on saving a little money. 



All of the above 
systems depended 
on one main feeder. 
Where there are a 
number of feeders 
originating at one 
main feeder panel, 
the matter is more 
simple. 

Assume a n i n e 
circuit feeder 



panel. 

Switch 1_25 hp. 

Switch 2. 15 hp. 

Switch 3_ 20 hp. 

Switch 4. 35 hp. 

Switch 5. 30 hp. 

Switch 6.40 hp. 

Switch 7. 60 hp. 

Switch 8.„. 60 hp. 

Switch 9_55 hp. 


Total .340 hp. 

50% equals 170 hp. 









































How to Figure D. C. Power Wiring 




This combination is practical and simple to execute. 


Switch 1 _ 25 hp. 

Switch 2 _ 15 hp 

Switch 3 _ 20 hp. 

Switch 4- 35 hp. 

Switch 5 _ 30 hp. 

Switch 6 _ 40 hp. 


165 hp. 


Switch 7 _ 60 hp. 

Switch 8 . 60 hp. 

Switch 9 _ 55 hp. 


175 hp. 


On a board of this character there are usually three 
horizontal sets of bus bars for three rows of switches, 
we therefore put the upper two on one side and the 
lower row on the other side of the 3-wire system. 

Where it is the intention to alternate from time to 
time from the 2-wire 115-volt system to a 3-wire 115- 
230 volt system, we proceed differently. 

First, the load must be carefully balanced as de¬ 
scribed above. The two main buses must be cut or dis¬ 
connected. The plant end of the buses must then be 
connected to one side of the double throw switch, the 
Edison service to the opposite side, and the feeder 
panels or load to the center legs. 


Plant 

■ -yj 


Edison 

+ 




±4 

+ 

H — 



Plant 

-1- 


A 


Edison 

+ 

_ 


V 


+ 



• X 


- 


Ordinarily, use is 
made of a 3 pole 
double throw un¬ 
fused switch, con¬ 
nected as shown 
here. 


For practical pur¬ 
poses it is simpler 
to connect as shown 
here. 


































56 


Profitable Power Wiring 


Two poles on the plant side are connected together 
by a piece of copper bus bar. 

Where 115 volt 2 
Eldison wire maximum cur- 

-— rent is 200 amps. 

-— the switch must be 

F 3 pole double throw 
200 amp. Where 
the 115 volt, 2 wire 
load is 400 amps, 
the switch must be 3 pole double throw 400 amps. 
Where it is 800 amps, the switch must be 3 pole double 
throw 800 amps., or it may be 4 pole double throw 
400 amps., which costs less, installs easier and operates 
as shown. 


Plant 














CHAPTER FIVE 


How to Analyze Load Preliminary to A. C* 
Power Wiring Work 


I F the alternating-current motor had the same start¬ 
ing characteristics that apply to the direct-current 
motor, there would be no need of repetition, because 
the wiring of direct-current motors is a simple matter; 
simply take the nameplate ampere reading and select 
a conductor to match, and it is reasonably certain that 
the work will be passed. 

With the alternating-current motor, though, condi¬ 
tions are entirely different. The starting current is 
far in excess of the running current, and great care 
and judgment must be exercised in every individual 
case, otherwise there may be too little copper, resulting 
in frequent fuse blowouts and starting difficulties or 
else a small fortune in copper conductors may be in¬ 
stalled way in excess of what would be needed. Some¬ 
times a 50-hp. motor operating a certain load can be 
wired on conductors that would be just right for a 
30-hp. motor operating a different load. 

For this reason a complete discussion of this matter 
would be interesting, and following is the procedure 
by which motor wiring layouts are prepared, and is 
the order in which this subject is to be discussed: 

Load to be operated and a complete analysis of the 
same. 

Motors available and an analysis of their character¬ 
istics. 



58 


Profitable Power Wiring 


Selection of type of motor to meet the requirements 
of the load and the central station supplying the 
power. 

Determination of wire, switch and fuse sizes. 

Approval of underwriters, municipal boards and 
central stations. 

It is necessary to proceed through all these various 
steps in order that one may be reasonably certain that 
his work will prove satisfactory, and that when com¬ 
pleted operation will- be perfect in every respect and 
will meet all necessary requirements and restrictions. 

The most important problem is not how much power 
is required to run the machine after it has once reached 
full speed, but how much power is required to start 
and put it into motion. If the power conditions at¬ 
tendant upon starting our load are known with a rea¬ 
sonable amount of certainty, the wiring system can be 
laid out very accurately. Therefore it is well to dis¬ 
cuss the various types of load and their starting char¬ 
acteristics before proceeding. 

The following types of machinery have the lighest 
starting load: 

Fans. 

Motor generator sets or generators. 

Machines controlled by friction clutches. 

Machines controlled by tight and loose pulleys. 

Short sections of shafting not exceeding 8 ft. and hav¬ 
ing no more than two hangers. 

Shafting controlled through friction clutches. 

Shafting controlled through tight and loose pulleys. 

The above loads involve the lightest possible start¬ 
ing effort, and an examination will reveal one, two or 
three of these outstanding characteristics: 

1. A small amount of static friction. 

2. A low weight of metal to be put into motion. 

3. A relatively low operating speed. 


How to Analyze Load 


59 


The combination of any one or all of these factors 
is equivalent to a low flywheel effect. It is putting into 
slow motion a relatively light mass of material with 
little static friction, and it will require a minimum 
amount of starting effort, an effort small in proportion 
to the total load carried when running under normal 
conditions. 

For the sake of contrast, there is listed below a num¬ 
ber of examples where the starting duty is heavy and 
severe: 


All woodworking machinery direct motor driven 
through belting or couplings. 

Machine shop shafting and power transmission sys¬ 
tems. 

Air compressors. 

Machinery equipped with flywheels. 

High speed pulverizers and grinders. 

Hoists, cranes and elevators. 

In each of these examples, any one, two or three of 
the following conditions exist: 

1. A high static friction. 

2. A great weight of metal to be put into motion. 

3. A relatively high operating speed. 

The combination of these conditions is equivalent 
to a heavy flywheel effect. It is, essentially, overcom¬ 
ing a heavy frictional inertia and putting into rapid 
motion a heavy mass of material which requires a 
maximum amount of starting effort, extremely large 
in proportion to the regular operating load. 

Table I shows clearly the approximate percentages 
of starting to full-load torque demanded by various 
types of power-driven machines. 

It might be well to explain what is meant by the 
terms torque, starting torque, full-load torque and 
their relation to horsepower. 


60 


Profitable Power Wiring 


Torque is the pull or turning moment required in 
applying power by rotation. It is expressed in pounds 

at one foot radius, sometimes called foot pounds. 

Starting torque is the turning moment a motor will 
develop in starting from rest. It is usually expressed 
in terms of full-load torque such as two times full-load 
torque, or 200 per cent of full-load torque, or 75 per 
cent of full-load torque. 

Full-load torque is the turning moment required to 
develop full-rated output of a motor of given horse¬ 
power at a given speed. 

Tr Full-L,oad Torque X r.p.m. 

Horsepower^-- 

The following tables give the full load torque of 
various sizes of motors operating at different speeds: 


♦Table I.—Approximate Percentages of Starting to Full 
Load Torque Demanded by Various Types of Power 
Driven Machines 


LIGHT 


MEDIUM 


HEAVY 


Machine Tools 


Boringmachines.... 

Metal saws. 

Sensitive and lathe 

upright drills. 

Buffing lathes. 

Emery wheels...... 

Speed lathes. 



Bolt cutters.f 

75 to 

Forming, bending [ 

175 


Milling machines. .\ 

1259! 

and straightening] 

to 

50 

Engine lathes.f 

75 

machines. [ 

300% 

to 

Slotting and key] 

to 

Heavy boring millsj 

150 to 

100% 

seating machines. [ 

1 5% 

25Q% 


Medium band saws| 

150 to 
175% 

Large radial drills../ 
t 

150 o 
200% 




Power hatnmers.../ 

175 to 




\ 

250% 




Heavy planers..../ 

200 to 




1 

300% 


T' 


Punching and Shea-f 

2C0 to 




ring machines.... \ 

L00% 
























How to Analyze Hoad 


61 


L'GHT 


MEDIUM 


HEAVY 


Stone Workers’ Machinery 


Marble drill press../ 

50 to 

Stone saws.] 

150 to 

Stone planers./ 

175 to 

\ 

100% 

Slate circular saws.\ 

200% 



250% 





Polishing stones 







and rubbing beds 


200 t(J 





Clay crushers. 


350% 





Pug mills. 







Tile grinders. 


175 tQ. 






i 

250% 


Woodworking Machines 


Jig saws. 


E ging saws. ( 

1 0 

Band saws and re-] 

200 to 

Pattern makers ' 

50 to 

Screw and dado] 

to 

saws. \ 

400% 

lathes. 

125% 

machines.1 

150% 

Large timber sizers] 

250 to 

Carvers. 


Buz* planers. / 

125 to 

and stickers. \ 

300% 



l 

175% 

Large planers, sur-f 

200 to 



Power fed glue join- [ 


facers and match-] 

300% 



ters.1 

125 





T noning and dowel] 

to 

Power feed door] 

175 to 



machines. 

150% 


250% 



Mortising machines ( 


Large multiplef 

225 to 



Panel raisers.-.... .] 

150 to 

drum sanders... .\ 

350% 



S rface s nders_ \ 

175% 

Large inside and] 

225 to 





outside moulders. \ 

275% 


Printing Machinery 


Tasting machines... 

50 

Job presses.] 

100 tc 

Quadruple and sex-f 

200 

Stitching machines..' 

to 


150% 

tuple newspaper] 

to 

Sewing machines.,. 
Book paging ma 

chines.. \ 

Tvpe casting ma-f 

cnines..... \ 

Envelope machines] 

100% 
75 to 
175% 
75 to 
175% 
75 to 
175% 

Cylinder and rotary] 

presses.it 

Linotype macnines] 

125 to 
175% 
125 to 
175% 

presses.1 

30G% 


Butchers’ and Grocers’ Machinery 



50 to 

Spice mills.] 


Meat choppers and] 

grinders.\ 

Lard presses.| 

150 to 

Filling machines....\ 

100% 

L*ard coolers.\ 

Coffee gri ders-] 

Food choppers.... \ 
Flour and taking] 
powder mixers. .. \ 

150%, 
150 to 
175% 
125 to 
15 % 

300% 
175 to 
250%, 


































































62 


Profitable Power Wiring 


LIGHT 

MEDIUM 

HEAVY 

Laundry Machinery 



Collar and bosom 
ironers. dampen¬ 
ers and starchers. 

125 

to 

150% 

Multiple roll man-/ 

gle.' 

Wringers or extrac-f 
tors.\ 

125 to 
260% 
250 to 
300%' 


Painters’ Machinery 



Miscellaneous 


Motor generator sets [ 

50 to 

Light line shafting/ 

100 to 

Ice cream freezers! 

200 to 

(direct drive).\ 

100 % 

\ 

175% 

(with cream frozen)\ 

300% 

Labelling and wiring f 

50 to 

Centrifugal pumps. / 

75 to 

Ice breakers./ 

175 to 

machines.\ 

100% 

\ 

150% 

i 

300% 

Buffing and grinding [ 


Refrigerating ma-f 

125 to 

Butter churns./ 

175 to 

machines.J 

50 to 

chinery.\ 

175% 

\ 

275% 

Direct connected] 

100% 



Carrier belts.f 


fans.[ 










Air compressors... 

200 





Piston pumps un- 

to 





derhead. 

300% 





Tumbler mills and 





agitators. 






Concrete mixers... 

175 to 





\ 

300% 





Coal crushers./ 

200 to 





\ 

300% 





Elevators andf 

250 to 





hoists.\ 

40J% 































































How to Analyze Load 


63 


EIGHT 

MEDIUM 

HEAVY 






Dough mixers.j 

Rubber grinders.. 

Sheet metal crim-f 
ping machines.... \ 
Corrugating rollf 

machines. \ 

Turn tables.j 

Steel mills for rail-/ 

ways......i 

Heavy main or| 

jackshafting.\ 

Electrically opera-f 
ted valves.\ 

200 to 
300% 
250 to 
300% 
250 to 
300% 
250 to 
300% 
250 to 
400% 
250 to 
400% 
200 to 
400% 
250 to 
400% 


Table II.—Full Load Torque of Motors in Foot-Pounds 
at Different Speeds 


Horsepower 

SPEED 

1800 r.p.m. 

1200 r.p.m. 

90) r.p.m 

1 

2.92 

4.375 

5.84 

2 

6.84 

8.75 

11.68 

3 

8.76 

13.125 

17.52 

5 

14.6 

21.9 

29.2 

7 H 

21.9 

32.85 

43.8 

10 

29.2 

43.8 

58. t 

15 

43.8 

65.7 

>7.6 

20 

58.4 

87.6 

1I6.8 

25 

73.0 

109.5 

146.0 

30 

87.6 

131.4 

175.2 

40 

116.8 

175 2 

333.6 

50 

146.0 

219.0 

292.0 


It is seldom necessary to measure running or full¬ 
load torque, but there are occasions where it is often 
very essential to determine the starting torque exactly. 
This one factor alone will frequently determine the 
type of motor that is required, the starting equipment 
to go with it, the size conductors needed and the type 
of protection demanded. It is necessary to know this 




























64 


Profitable Power Wiring 


in order that it be possible to comply with the rules 
and regulations of the underwriters, the municipal au¬ 
thorities, the central station and finally for the satis¬ 
faction of the customer. 

Starting Torque Must Be Calculated 

It becomes necessary to get this data in the follow¬ 
ing instances, and although the horsepower require¬ 
ments may be evident, it will frequently be found that 
the motor is unable to start the load unless the start¬ 
ing characteristics are known. 

A—Where the equipment has been operated from 
one prime mover, such as a steam engine, and where 
this engine will be replaced by either one motor or a 
number of small motors. In this situation the horse¬ 
power requirements can usually be accurately ascer¬ 
tained in advance, but not so as to starting effort. 

B—Where mechanical power has been purchased 
and supplied through a belt, and where a motor is in¬ 
stalled to replace mechanical with electrical energy. 

C—Where it is. the intention to install individual 
motors on machinery formerly operated from shafting 
through tight and loose pulleys. 

D—A new type of machine to be designed and man¬ 
ufactured. Here, of course, the manufacturer makes 
all the necessary tests and is able, or at least he should 
be able, to tell the contractor the essential data. 

In the majority of instances there is little need of 
making experimental tests to determine the amount of 
starting torque compared with full-load running torque, 
since much data have been compiled upon that subject. 
For example, it is known that fans, blowers and motor 
generator sets have the lightest and smallest possible 
starting torques and that elevators and heavy power 
transmission systems have the heaviest possible start¬ 
ing torque. These examples constitute two extreme 
cases of light and heavy starting torque. In the case 


'How to Analyze Load 


65 


of light starting apparatus it is seldom necessary to 
make tests for actual values, but in the case of heavy 
starting duty it is well worth while to go into the mat¬ 
ter in more detail, for it will not only save the con¬ 
tractor much trouble and expense, but will give him 
positive assurance that whatever apparatus he selects 
it will work and will render satisfactory service. 

Determining Starting Torque 

Determining actual starting torque requirements is 
a simple matter. It can be accomplished without the 
aid of expensive laboratory apparatus and can be done 
in a very few minutes. If, for example, we wish the 
starting torque of a machine shop transmissions system 
to be determined, proceed as follows: 

1. Remove main power belt. 

2. Lash a plank or crowbar to main pulley. 

3. At a specified distance measured horizontally 
from the center of the shaft—say 10 ft.—slowly add 
weights. 

4. These weights are slowly increased until an 
amount is reached where the shaft begins to turn. Re¬ 
peat this test to be sure that results are correct. Say 
the weight added amounts to 50 lb. Then the starting 
torque required will be 50 X 10 = 500 ft. lb. 

It is known that the operating load is 50 hp., and 
that a 50-hp., 900 r.p.m. motor can readily be used. 
From Table II on full-load motor torque, it is seen 
that the full-load torque of this motor is 292. The 
maximum starting torque of a squirrel-cage motor of 
that rating is 125 per cent of full-load torque, or 292 
X 1.25, which equals 365 ft. lb. Since, however, 500 
ft. lb. are needed for starting, a squirrel-cage motor 
will not do, and slip-ring type must be used which can 


66 


Profitable Power Wiring 


develop as high as 350 per cent starting torque, or 292 
X 3.5, which equals 922 ft. lb., or almost twice as much 
as needed. 

An instance which came to the writer’s attention 
proved how very useful such data could be. It was a 
question of motorizing a special machine, which was 
used for manufacturing cable. This machine had been 
run from iine shafting on a tight and loose pulley and 
tests made on the motor showed a load of 3.9 hp. It 
was then determined to use a. 5-hp. motor, squirrel- 
cage, to drive this machine. 

The question arose, What would happen if this ma¬ 
chine were stopped in the middle of the coil and had to 
be started again? Would the motor be able to do it? 
At first—it seemed almost absurd to ask that question 
—but when a doubt exists, there is no need to take 
chances—particularly in this case, where 20 machines 
were to be motorized. It was decided to make a test 
for starting torque under the worst conditions. The 
machine was run until the coil was almost complete 
and then stopped. A crowbar was lashed to the driv¬ 
ing pulley and a string with a pail tied to it at a point 
4 ft. from the center of the shaft. Adding weights 
gradually, it started to move when 8 lb. were in the 
pail. The starting torque, therefore, was 8X4, or 32 
ft. lb. A 5-hp., 1800 r.p.m. squirrel-cage motor has a 
running torque of 14.6 ft. lb. and a starting torque of 
150 per cent, or 14.6 X 1.5, which equals 21.9 ft. lb. 

The test, therefore, proved conclusively that a 5-hp. 
motor would not start this machine unless a tight and 
loose pulley were used, and by making this preliminary 
test a great amount of unpleasant trouble was avoided. 

The reason why the authorities are unable to state 
definitely what size conductors are to be used in wir¬ 
ing for an alternating current power is because they 
do not know the characteristics of the load the motor 


How to Analyze Load 


67 


is to drive. Rather than take any chances, they will 
recommend a size conductor that is greater than the 
size actually needed and is heavy enough to provide 
for the worst conditions. 

Having determined the starting torque, the next step 
is to study the characteristics of the motors available, 
and then select the type to meet the requirements of 
the load and the central station supplying the power. 
This subject will be discussed in great detail in the 
next chapter. 

























































































































. 








. 


















* 










































CHAPTER SIX 


How to Select the Correct Type of 
A. C. Motor 


I N laying out alternating current power work it is 
essential not only that the starting and full load 
characteristics of a piece of machinery be known (as 
was explained in the previous chapter), but also that 
a decision be made as to whether single phase or 
polyphase motors be used and of what types. 

Polyphase motors may be roughly divided into two 
broad classes: 

1. Squirrel-cage type. 

2. Wound rotor type. 

The squirrel-cage motor is about the simplest motor 
mechanism known. It consists of a wound stationary 
laminated steel field frame, called the stator and a 
laminated steel and copper rotating element called the 
rotor. 

If we throw this motor directly across the line, 
phenomena indicated in Table I will take place, regard¬ 
less of horsepower. 

Just what these figures mean is this. If an 1800 
r.p.m. motor is thrown across the line it will develop 
150 per cent of full load torque and will draw six 
times full load running amperes. 

If, for instance, the load is 5-hp. with service at 
220 volts, 3 phase, 60 cycles, where full load motor 
running amperes are 14, inrush whether the starting 
torque be light or heavy, will be 14 x 6 or 84 amp. 



70 


Profitable Power Wiring 



A Slip Ring Motor 

On the side opposite the pulley end can be seen the slip rings and brush 
holder rigging and brushes. Resistance is introduced into the rotor 
circuit through these rings. 

Courtesy General Electric Co. 


In the case of a light load this inrush is only for a 
fraction of a second, and if 30 amp. fuses are in¬ 
stalled they will hold because of the lack of time neces¬ 
sary to heat them sufficiently so that they will melt. 
The fact that a 30-amp. fuse will not blow is no evi¬ 
dence that the current is not over 30 amp. It simply 
means that the time factor is too short. In the case 
of a heavy load, where the motor will not start,the fuses 
will blow almost instantly, due to the continuous over¬ 
load. The duration of this inrush of six times full 
load running amperes depends upon the type of load 
to be started. 

Therefore, in wiring for polyphase squirrel-cage 
motors, the known factors are: 

1. Full load running amperes. 

2. Maximum starting-torque is 15 per cent of full 
load torque. 


How to Select Motor 


71 


3 Inrush is six times the full load running amperes. 
The unknown factors are: 

1. Length of time required for acceleration. 

2. Time element of fuse. 

Common practice calls for a fuse and conductor 
rated at 2^2 times full load running amperes. Where 
the starting load is light and medium, these will hold. 
Where, however, the starting torque is heavy, a fuse 
3 Yz to 4 times full load amperes is recommended. 


Table I—Starting Characteristics of Squirrel-Cage Motors 


Hp. of 
Motor 

Speed of 
Motor 

Starting Torque 
in Per Cent 
of Full 
Load Torque 
Developed 

Starting 
Current Inrush 
in Per Cent of 
Full Load Amps. 

Actual 

Amperes 

Drawn 

5 

1,800 

150 

600 

84 

5 

1,200 

135 

550 

77 

5 

900 

1Z5 

500 

70 

5 

600 

115 

460 

65 


Table II—Starting Characteristics for 25 HP. Motor 
at Various Compensator Taps 


Starting 

Conditions 


Across the line 
100% voltage 

85% tap. 

70% tap. 

58% tap. 

40% tap. 


Starting 

Torque Starting . Actual 

Per Cent Amps, in Starting 

of Full Per Cent Full Actual Torque in 
Load Torque Load Amps. Amps. Foot Pounds 


150 

600 

360 

109 

128 

433 

260 

93.5 

105 

294 

176 

76.5 

51 

203 

122 

34.2 

24 

96 

58 

17.5 


Where polyphase squirrel-cage motors in size over 
5-hp. are used, it is customary to use an auto starter 
or compensator to reduce the inrush of current while 
starting. 
















72 


Profitable Power Wiring 


It is most essential that this be done, for, if a 25-hp. 
1800-r.p.m., 220-volt, 2-phase, 60-cycle motor, for in¬ 
stance, was thrown directly across the line, the inrush 
would be 60 X 6 or 360 amp., which would not only 
require very heavy conductors, but would cause a seri¬ 
ous voltage drop, which would reduce starting torque 
and bring on other serious difficulties, complications 
and troubles. 

An autostarter or compensator is an auto-transform¬ 
er with several taps taken from the windings to pro¬ 
duce different voltages for starting. 

From one manufacturer’s data book on compen¬ 
sators for motors 20-hp. and over there are four taps. 
These produce 40, 58, 70 and 85 per cent of line voltage. 
By the use of these various taps it is possible to secure 
the results shown in Table II for a 25-hp., 1800-r.p.m., 
220-volt, 3-phase, 60-cycle motor. 

It is evident, then, that it is possible with the same 
motor, under five separate conditions, to secure five 



Rotor of a Wagner Polyphase Self-Starting 
Type Motor 

This rotor has two separate windings, one of which is submerged by the 
other. The submerged winding is of the squirrel cage type, the other 
outside winding is of the wound type, of high resistance and terminates 
in a commutator. At one end is the centrifugal governor mechanism 
which in this motor acts as follows: Upon start, the high resistance 
winding produces a high torque with a low ampere inrush and as the 
motor comes up to normal speed, the centrifugal governor acts and 
short circuits this high resistance winding, which then becomes in 
effect a low resistance winding in multiple with the squirrel cage 
winding, and the motor then operates as a straight polyphase squirrel 

cage motor. 

Courtesy Wagner Electric Manufacturing Co. 









How to Select Motor 


73 


different values of starting inrush varying all the 
way from a little less than full load amperes to fully 
six times full load running amperes. Where the full 
load current is 60 amperes, the current inrush will vary 
all the way from 58 to 360 amperes, or from 96 to 600 
per cent full load nameplate amperes. 

This is but a further reason why it is practically im¬ 
possible for any authority to say just what size conduc¬ 
tor to use. The contractor, however, may determine 
by actual test or from data available what the starting 
torque requirements are. 

These compensators as delivered from the manufac¬ 
turer are usually connected to the 58 per cent tap. 
The usual procedure to determine the correct tap is 
this: 

After the motors are set and wired completely, start, 
then, to find out if the direction of rotation is correct. 
Next, belt the motor to the machine and start the motor 
again. If the motor rotor remains stationary, the 



Rotor of a Slip Ring Motor 

It carries a winding which consists of three sections interconnected 
through the slip rings and outside resistance. By introducing ohmic 
resistance in series with this winding, we can vary the torque at different 
speeds and we can also vary the speed of the motor by means of this 
resistance. 

Courtesy General Electric Co. 

current is insufficient, so throw it off and change the 
compensator tap to the next higher. If again nothing 










74 


Profitable Power Wiring 


happens on starting, try the next higher tap. This 
time the machine may start; but the fact that it was 
necessary to change taps twice spells trouble; for, 
within a few days, the representatives of the central 



Interior o£ a Compensator 

One of the functions of this piece of 
apparatus is to reduce impressed voltage 
upon starting to limit the starting in¬ 
rush of current. This is accomplished 
by means of an auto-transformer. This 
compensator is for three phase service 
and the three coils of the auto-trans¬ 
former can be seen immediately over 
the “Danger” sign. The auto-trans¬ 
formers in starting compensators are so 
wound that it is possible to change the 
connections and produce various volt¬ 
ages for starting by changing from one 
tap of the winding to another. These 
different taps are brought out at the top 
of. the coils and can be plainly seen. It 
is just a moment’s work to reconnect 
from a higher to a lower or vice versa. 

Courtesy General Electric Co. 


station will test for starting ampere inrush, and will find 
it beyond the limit allowed. The central station will 
then notify the consumer to this effect and request 
that the necessary changes be made so that this motor 
will start the load without violating the rules and regu¬ 
lations of the company in regard to motor current in¬ 
rush. If this notice is not carried out, the central sta¬ 
tion advises that the supply of electric power will be 
discontinued until such time as the necessary changes 
have been made. 

The contractor who is familiar with the starting 
characteristics of various kinds of machinery would 
not allow himself to be placed in that awkward pre- 


How to Select Motor 


75 



Rotor of a Single Phase Motor 

It shows a winding on the rotor and from the end shown can be seen 
the centrifugal governor mechanism. At the opposite end is a commu¬ 
tator and a set of brushes, which short circuit several bars. Upon 
closure of the line switch the motor starts as a repulsion motor and as 
it approaches full speed, the centrifugal switch mechanism operates and 
performs two functions—one, short circuiting the commutator; two. 
removing the brushes from the commutator, so that the motor operates 
as a straight squirrel cage induction motor. 

Courtesy Wagner Electric Manufacturing Co. 

« 

dicament. All central stations have rules and regula¬ 
tions governing the motor apparatus that is installed 
on their lines, and every contractor should know them 
thoroughly. If he is in doubt, he should take a half a 
day off and discuss it personally with the official or 
engineer who has direct charge of this matter. The 
time spent in this manner is more profitable than rip¬ 
ping out an entire job and replacing motors. Motors 


Table III—Sfarting Torque and Current for Each 
Starter Tap 




Starting Torque 
in Per Cent of 

Full Load Torque- 

Starting' 
Inrush Amperes 
in Per Cent of 
Full Load Amperes 

1st 

Step 

25 

25 

2nd 

Step 

50 

50 

3rd 

Step 

75 

75 

4th 

Step 

100 

100 

5th 

Step 

125 

125 

6th 

Step 

150 

150 

7th 

Step 

175 

175 

8th 

Step 

200 

200 














76 


Profitable Power Wiring 


that were new are then second-hand, not to say any¬ 
thing of the time and expense and annoyance suffered 
by the customer. 

On polyphase motors over 5-hp., the rules and regu¬ 
lations of central stations vary, but in most instances, 
except for very large motors, they permit an inrush 
of not over 250 per cent of full load running amperes. 
From Table II it is seen that the compensator may 
be connected only on the 40 and 58 per cent taps to 
meet this requirement, the former giving a starting 
torque of 24 per cent of full load running torque, and 
the latter 51 per cent. Now, if this motor is to drive a 
fan, a blower or a motor generator set, 24 pet cent 
torque is sufficient, but if it is to run shafting or trans¬ 
mission, particularly if it is to be started with all 
belts on the tight pulleys, there is less certainty. Under 
such circumstances, measure the torque. The test will 
only take fifteen minutes, and will show whether or 
not it can be done. If results say no, use a different 
type motor, or else operate it from a tight and loose 
pulley or friction clutch. It is not necessary to guess. 

If it is found that the starting torque required is less 
than 24 per cent of full load running torque, the motor 
can be connected to the lowest tap of the compensator. 
At this point the starting current will be less than full 
load running amperes, and the conductors and fuses 
need not be heavier than the full load rated amperes 
of the motor. 

The writer has on several occasions installed 50-hp. 
squirrel-cage motors where the starting duty was ex¬ 
tremely light. In each case these motors were ap¬ 
proved by the central station, and in every instance 
the wire sizes were sufficient to carry only the full 
load amperes of the motor, which sizes were later ap¬ 
proved by all of the inspection boards. 

It does no harm to consult with the motor manufac¬ 
turers on cases of this character to secure data on 


How to Select Motor 


77 


starting inrush. Their extensive knowledge will be of 
very material assistance in obtaining the data needed. 

High Resistance Rotor 

There is a special squirrel-cage motor which differs 
somewhat from the standard type. It is called a high- 
resistance rotor squirrel-cage motor. It differs from 
the standard type in that the rotor has a higher resist¬ 
ance than that of the ordinary squirrel-cage rotor. 
This added resistance is obtained by using silver solder 
for connecting the rotor bars to the end rings, or else 
by using a different metal composition in the cast end 
rings. One effect of this added resistance is to pro¬ 
duce a higher starting torque with proportionately low¬ 
er current inrush, while another is to cause a greater 
drop in speed upon a sudden increase of load. 

This type of motor is employed for elevator service 
and on machines equipped with heavy fly wheels, such 
as punch presses, bolt cutters and shears. It is not 
commonly used in sizes over 15-hp. where for certain 
reasons the wound rotor type of alternating current 
motor is employed. 

Wound Rotor Type 

The wound rotor motor differs from the squirrel- 
cage type in that the rotor has a definite and distinct 
three circuit winding on it, whereas the squirrel-cage 
rotor, true to its name, has a series of copper bars in 
slots, short-circuited at both ends by end rings, either 
cast, soldered, bolted or riveted. This wound rotor 
winding permits the insertion of ohmic resistance in 
the rotor circuit which serves several purposes: 

1. It increases the torque at different speeds. 

2. It reduces the amount of amperes drawn in pro¬ 
portion to the torque developed. 

3. It permits variation in motor speed. 


78 


Profitable Power Wiring 


This external resistance is introduced in different 
fashions, in the two types of wound rotor motors: 

1. The internal resistance type. 

2. The slip ring type. 

In the internal resistance type, the resistance is 
mounted within the rotor and is cut in and out of the 
rotor circuit by means of a centrifugal switch mounted 
upon the rotor shaft. On starting, all resistance is in 
circuit, and upon reaching a certain predetermined 
speed, the switch operates and short-circuits the resist¬ 
ance. By this device the motor has the starting char¬ 
acteristics of the slip ring type and the operating char¬ 
acteristics of the squirrel-cage type. 

The internal resistance motor is installed without 
the use of any starting device except a knife switch 
or an automatic contactor. Upon closure of the circuit 
the motor will start up at once and accelerate to full 
load speed, unless the starting torque required is in 
excess of that developed by the motor. This type will 
produce ordinarily a starting torque equal to 200 per 
cent of full load torque and the current inrush will 
correspond to 250 per cent, or 2times full load 
running amperes. 

Thus, the wiring of this type motor is a simple mat¬ 
ter. Multiply nameplate amperes by 2 y 2 and select a 
conductor to match this current. 

If the starting torque is extremely light, the motor 
will accelerate more rapidly, and the duration of the 
inrush of current will be appreciably lessened. 

In the slip ring type the three circuits are brought 
out through three collector rings mounted on the shaft 
of the rotor, and the resistance may be in the form of 
cast grids with drum control or incorporated in a face¬ 
plate dial controller. 

The percentage starting torque and percentage start¬ 
ing inrush amperes are practically the same, as per the 
internal resistance type. It is possible, however, 


How to Select Motor 


79 


through the selection of external resistance to meet 
almost any condition imposed. Standard practice to¬ 
day calls for the introduction of 7 or 8 steps of resist¬ 
ance, which on closing the circuit will develop the 
starting torque and current inrush shown in Table III. 

It is an arbitrary table, but the values may be ob¬ 
tained with any slip ring motor. Usually it is consid¬ 
ered fair and good practice to allow 150 per cent start¬ 
ing amperes for all slip ring motors. For with that 
value a starting torque is obtained that would corre¬ 
spond to the torque developed by the squirrel-cage 
motor if thrown directly across the line without any 
intermediate starting and control apparatus and with 
an inrush of current one-fourth as great. This is the 
real reason why central stations want their customers 
to use slip ring motors. A maximum amount of start¬ 
ing torque is obtained for a minimum amount of cur¬ 
rent drawn from the line. 

It is, therefore, standard practice among larger cen¬ 
tral stations to demand slip ring motors in all cases 
for motors over 20-hp. in size. 


Single Phase Motors 

No space as yet has been devoted to single-phase 
motors because most of these are in small sizes. The 
most popular type of single-phase motor in use today 
is the repulsion type starting with the centrifugal switch 
to cause it to operate as a squirrel-cage motor at full 
speed. For starting direct across the line, current in¬ 
rush is four times full load running current, corre¬ 
sponding to 200 per cent full load torque. For start¬ 
ing through a rheostat, current inrush is two times 
full load running current, corresponding to 50 per cent 
full load torque. These motors are wound so that 
they may be run on 110 or 220 volts by connecting the 
four outcoming leads either in parallel or in series. 


80 


Profitable Power Wiring 


To summarize a bit, it can be said that, the reasons 
why it is impossible for the various authorities to state 
definitely what size conductors are to be used in the 
wiring of alternating-current motors are because: 

1. Every machine or group of machines to be driven 
by a motor is a separate and distinct problem in 
itself. 

2. The amount of starting torque required is a vari¬ 
able quantity, which is due to: 

a—Temperature changes from summer to winter, 
causing a variation in the viscosity of lubri¬ 
cating oils, and which affects the static fric¬ 
tional inertia. 

b—Wearing down of machine bearings, 
c—Variation in tension of belting, 
d—Change from experienced to inexperienced 
machine operators. 

e—Change in the character of materials fed into 
the machine. 

The preceding description of the various types of 
a.c. motors available and their outstanding starting 
characteristics brings these conclusions: 

For motors, constant speed, 5-hp. and less, the squir¬ 
rel-cage and the single-phase types are used universally. 

For motors, constant speed, 7j4 to 20-hp., the squir¬ 
rel-cage type is used with compensator or autostarter. 

For motors, constant speed, 20-hp. and above, squir¬ 
rel-cage and wound rotor motors are used. 

Squirrel-cage motors are cheaper, simpler and re¬ 
quire less attention. 

Wound rotor motors are used mainly because cen¬ 
tral stations impose limitations on current inrush, and 
this type of motor is the only one that will meet these 
conditions and at the same time provide sufficient 
starting torque to accelerate the load. An example 
will illustrate this. 


How to Select Motor 


81 


Motorizing a Woodworking Plant 

A woodworking plant is to be motorized. There is 
one 56-in. band resaw which requires 50-hp. and is to 
be driven direct from the motor. There are two 
planers operated from a short jackshaft through tight 
and loose pulleys that require a total of 50-hp. and are 
to be driven by one 50-hp. motor. 

A typical instance of a central station ruling is illus¬ 
trated in the paragraph below, taken from an actual set 
of regulations for 220-volt, 2-phase, 60-cycle service: 

“Motors of over 15-hp. capacity must be of the 
wound rotor type with external or internal starting 
resistance which will start the motor under actual load 
conditions without exceeding 5 amp. per phase per 
rated horsepower. In an installation where the mo¬ 
tor starts without load a squirrel-cage motor will be 
accepted provided the starting current does not ex¬ 
ceed that mentioned above for a wound rotor motor.” 

Full load amperes on a 2-phase, 220-volt, 50-hp. 
motor are 108 per phase. Inrush amperes are limited 
to five per horsepower per phase, or in this case to 
250 amp. 

Consider the first application, the 52-in. band resaw. 
Here the motor is either belted direct to the saw or 
drives it through a flexible coupling. This machine 
requires a heavy starting torque, as will be seen from 
consulting Table I in the preceding chapter under 
“woodworking machines/’ which gives band saws and 
resaws at 200 to 400 per cent full load torque on start¬ 
ing. This means that it will require from two to four 
times full load torque to start this machine. The 
squirrel-cage motor at its best, if thrown across the 
line without a starter, will only develop a starting 
torque of 150 per cent, and is therefore of no use for 
this service. 

The choice then lies between the wound rotor inter¬ 
nal resistance type and the slip ring type. The starting 
characteristics of the internal resistance type motor 


82 


Profitable Power Wiring 


show a torque of 200 per cent, corresponding to 250 per 
cent inrush full load amperes. But as the starting re¬ 
quirements of a band resaw are from 200 to 400 per 
cent full load torque, to use this type of motor would 
be courting trouble. It might start the machine or it 
might not. Where conditions are doubtful, play safe. 
The slip ring type is the only one to be used that will 
meet all conditions as to starting inrush of current 
and starting torque. 

Two Planers Operating on One Jackshaft 

Having settled this motor application, consider the 
other one, the two planers operating from one jack- 
shaft. 

The operating load will be 50-hp. Both planers will 
be operated from the jackshaft. They will be started 
by shifting an idler pulley in such fashion that the 
slack in the driving belt will be gradually absorbed 
until the machine is running at full speed, when the 
idler pulley will wrap the belt securely about the 
driven pulley. Therefore, to start, this motor will 
have to put into operation a 2 15/16-in. shaft 15 ft. 
long, running on three hangers and having three pul¬ 
leys thereon, with only one tight belt, the one from 
motor to shaft. The shaft operates at a speed of 200 
r.p.m. An examination showed that it was possible to 
turn this shaft with the slightest pressure of the hand on 
the pulley. All of these conditions correspond with a 
light starting torque. 

A 50-hp., 900-r.p.m. motor is needed. Full load 
torque is 292 ft.-lb. The lowest tap on the compensa¬ 
tor would develop 24 per cent of full load torque, or 
73 ft.-lb. The inrush to correspond would be 96 per 
cent of full load amperes, or 103. The second lowest 
tap on compensator would give 51 per cent of full load 
torque, or 150 ft.-lb. The inrush to correspond would 
be 203 per cent of full load amperes, or 222. The cen- 


How to Select Motor 


83 


tral station permits 250 amp. inrush. Hence it all de¬ 
pends whether the shaft can be started on either of 
these compensator taps. 

Referring to Table I in the preceding article, it is 
seen that motor generator sets can be started with 50 
per cent full load running torque, and as this load 
is very similar to such a load, a squirrel-cage motor 
is tried. Upon connecting for the first tap the motor 
starts with ease and without difficulty. 

The central station makes a test, but the inspector 
suggests connecting to the second tap, and in order 
that it start more rapidly, for even on the second tap 
the starting current would not exceed the minimum 
requirements. 

The installation of this type of motor, therefore, 
saves the contractor fully 15 to 20 per cent on his 
cost, helps him very materially in securing the contract, 
and at the same time makes a good, sound installation. 

Having explained the method to be pursued in se¬ 
lecting an alternating-current motor, the next step is 
to tell how to select the proper size of conductor. In 
the planning and design of the installation, it is neces¬ 
sary to consider conductors, iron conduits, switches, 
fuses and other protective devices. 

Single-phase motors, as stated above, when thrown 
across the line without starters, draw four times full 
load running current. When started through a rheo¬ 
stat they draw two times full load running current. 

Polyphase squirrel-cage motors when started through 
autostarters or compensators draw anywhere from full 
load current to four times full load amperes, depend¬ 
ing on what tap they are connected. 

Wound rotor internal resistance motors when thrown 
directly across the line draw 2]/^ times full load run¬ 
ning current. 

Slip ring motors when started through faceplate or 
drum controllers draw anywhere from J4 to 3times 


84 


Profitable Power Wiring 


full load current, depending upon the starting charac¬ 
teristics of the load. 

Data Put Into Rule Form 

Put into rule form, these data may be stated about 
as follows: 

With single-phase motors for heavy starting duty, 
where starting torque is from 150 per cent up without 
starters, multiply nameplate amperes by 2^ and install 
switch and fuses to correspond to this value. For the 
proper size of conductor refer to Table C of Under¬ 
writers and use nearest larger size. 

With single-phase motors for light starting duty 
where starting torque is 50 per cent to 150 per cent 
with starting rheostat, multiply nameplate amperes by 
2 and install switch and fuses to correspond to this. 
For the proper size of conductor refer to Table C of 
Underwriters and use nearest larger size. 

By referring to Table I in the preceding chapter the 
classification under which the load comes will be 
found, whether light or heavy starting. If in doubt, 
use the heavy rating. 

Polyphase squirrel-cage motors of 5-hp. or less, 
when thrown across the line draw 6 times full load 
running current. The length of this inrush depends 
upon the character of the load to be started, and varies 
from 1/3 to 3 seconds. A good safe rule is to multiply 
the nameplate amperes by 2y 2 . Refer to Table C of 
Underwriters Code and select the nearest larger size, 
installing fuses and switch to correspond. 

This will work out satisfactorily except where the 
starting conditions are severe and where frequent 
start and stops are required. While a wire and fuse 
to carry 2 y 2 times full load running are not sufficient 
to carry the inrush of six times full load running am¬ 
peres continuously, the time is of such brief duration 
that the fuse will not get hot and blow out. But where 


How to Select Motor 


85 


the starting load is severe and where starting and stop¬ 
ping is frequent, say once every three minutes, then 
the fuse will blow. Therefore, under such extreme 
conditions it is advisable to multiply the nameplate 
amperes by four, and select conductors, fuses and 
switch to match this value. 

For polyphase motors 7 1 / 2 -hp. and over, started 
through compensators, the starting inrush may vary 
anywhere between full load amperes to four times full 
load amperes. As it is customary for the central sta¬ 
tion to limit inrush to approximately 2y 2 times full 
load running amperes, it permits the use of only the 
two lowest starting taps of the compensator. It is, 
therefore, entirely up to the contractor to determine 
whether or not the motor will start the load under 
these conditions. Under the worst circumstances, the 
current inrush will be 2 y 2 times full load amperes. 

How to Obtain Sizes 

Therefore, to obtain size of wire, switch and fuses^ 
multiply nameplate amperes by 2 y 2 , and select nearest 
larger size cable from Table C of Underwriters. 

If, however, the analysis of the load shows it can 
be started on the first tap, switches, fuses and con¬ 
ductors, equivalent to full load nameplate amperes may 
be used. These conductors, however, should be se¬ 
lected from Table A and not from Table C, because 
Table C rating is permitted only for inrush, whereas 
Table A is for continuous duty. 

For wound rotor motors internal resistance type, 
multiply nameplate amperes by 2 y 2 and select switch, 
fuses and conductors from Table C of Underwriters. 

For slip ring motors multiply nameplate amperes by 
\y 2 and select switch, fuses and next larger conduc¬ 
tor from Table C of Underwriters to correspond. For 
the size of conductors from slip rings to external re- 


86 


Profitable Power Wiring 


sistance apply to manufacturer and select conductors 
from Table A to correspond. 

On all alternating-current conduit-wiring installa¬ 
tions it is essential that all wires of a circuit be carried 
in one pipe: For single-phase, 2 conductors per pipe; 
for 3-phase, 3-wire, 3 conductors per pipe; for 2- 
phase, 4-wire, 4 conductors per pipe; for 2-phase, 
3-wire, 3 conductors per pipe. 

For single-phase, 2 conductors per pipe. 

For 3 phase 3-wire, 3 conductors per pipe. 

For 2 phase, 4-wire, 4 conductors per pipe. 

For 2 phase 3-wire, 3 conductors per pipe. 

It seems somewhat absurd to have to put this rule 
in writing, but the writer knows a case where one of 
the largest contractors in the country disregarded it 
and was put to considerable expense in changing it 
over. It might be well to add that conductors in all 
the systems mentioned above are of the same size ex¬ 
cept the 2-phase, 3-wire. In this instance the third or 
common wire is derived by combining into one con¬ 
ductor one wire of each phase, and the current in it 
is equal to 1.41 times that of each of the other outside 
wires. Therefore, in all such instances multiply the 
amperes obtained for the outside conductors by 1.41 
and select the common wire in accordance with this 
increase. 

The foregoing data applied to wiring runs for indi¬ 
vidual motors only. 

Calculating Size of Feeders 

We now will take up briefly the design of feeders 
carrying two or more alternating-current motors. It 
is possible, but not probable, that all the motors will 
be started at the same identical moment. Therefore, 
starting requirements form only a minor considera¬ 
tion in the selection of feeder conductor sizes. It must 
be remembered, however, that all ratings for conduc- 


How to Select Motor 


87 


tors for feeders should be based on Table A ratings 
throughout. The proper procedure is as follows: 

Add up the nameplate rating of all motors, except 
the largest one, to be fed from this feeder. Add to 
this total the starting amperes of the largest motor, 
obtained as shown previously. This last total gives the 
amperes for which conductors should be selected from 
Table A. 

The theory of this method is that the worst possible 
condition would be, where all motors were operating 
at full load, when the largest motor was thrown across 
the line. 

It must be understood that this rating does not take 
into account either load factor or diversity factor. 
Load factor meaning that all motors are not operating 
at full rated load, and diversity factor meaning that 
all motors are not operating at the same time. How¬ 
ever, load factor will have little to do with cutting 
down the size of the feeder, since alternating-current 
motors operating at less than full load take very nearly 
the same amount of amperes due to lower power fac¬ 
tor. Moreover, unless it is a tremendously large in¬ 
stallation, and the first cost is of vital importance, it 
does not pay to skimp on a.c. motor conductors. 

Nothing has been mentioned about drop in voltage. 
It must be remembered that the torque of alternating- 
current motors varies as the square of the voltage. If 
the torque is 100 per cent at full voltage, a drop in 
voltage of 10 per cent will cause the torque to decrease 
to 81 per cent. And the heavy inrush ampere of small, 
low-speed motors that have a poor starting torque at 
the best, cause a drop in voltage. If the run is long 
and the conductors are skimped, this voltage drop will 
affect the starting torque and prevent operation. 



CHAPTER SEVEN 


How Many Power Panels Should Be Used? 


I N the wiring and preparation of this series of 
articles on the design of power wiring it has been 
found that it is impossible to prepare a set of rules 
and regulations that will apply to every piece of work. 
Consequently it was decided to start at the beginning, 
suggest a tentative scheme and work it out. Then by 
proposing other further schemes and working them out 
results may be compared and conclusions drawn. Such 
a procedure will be followed in this chapter, which is 
devoted to the subject of wiring for two-phase. 

The example taken is an iron works which had been 
operated from a steam engine and was to change over 
to electric drive with 220-volt, 60-cycle, 4-wire, two- 
phase service. The sketch shows the plan of the build¬ 
ings and the motor layout. 

There are seventeen motors, with a total rating of 
23 7 horsepower. At 2]/z amp. per horsepower there is 
a total current of 592 amp., or, roughly, 600 amp. 

Before preparing the data table, first look up the 
rules and regulations of the central station that supplies 
the power. They will be somewhat as follows: 

Under 5 Horsepower 

Motors of less than 5 hp. may be connected without 
any starting device provided the starting current does 
not exceed 14 amp. per phase per rated hp. 

5 to 15 Horsepower 

Motors from 5 to 15 hp., inclusive, must be equipped 
with an approved starting device which will start the 



90 


Profitable Power Wiring 



FLOOR 
BLDG. "A 


GROUND OR 
jai FLOOR 


HEIGHT-FLOOR TO FLOOR— IB'-O” 
WOOD FRAME CONSTRUCTION 




Plan Showing Location of Motors 



















































































How Many Power Panels Should Be Used? 


91 


Item No. 

K Squirrel Cage, 

2 Squirrel Cage, 

3 Squirrel Cage. 

4 Squirrel Cage. 

5 Squirrel Cage. 

6 Squirrel Cage. 

7 Slip Ring. 

8 Squirrel Cage. 

9 Squirrel Cage. 

10 Slip Ring. 

11 Squirrel Cage. 

12 Slip Ring. 

13 Squirrel Cage. 

14 Squirrel Cage. 

15 Squirrel Cage. 

16 Squirrel Cage. 

17 Squirrel Cage. 


.15 Hp. 
15 Hp. 
15 Hp. 
15 Hp. 
1 Hp. 
3 Hp. 
50 Hp. 
15 Hp. 
15 Hp. 
20 Hp. 
7% Hp. 
25 Hp. 
5 Hp. 
3 Hp. 
10 Hp. 
15 Hp. 
7% Hp. 


900 

900 

900 

900 

1800 

1800 

900 

900 

900 

900 

1800 

900 

1800 

1800 

900 

900 

1200 


R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

R.p.ra. 

R.p.m. 

R.p.m. 


Shafting 
Shafting 
Shafting 
Shafting 
Sump Pump 
Hydraulic Press 
Compressor 
Shafting 
Shafting 
Power Brake 
Motor Generator 
Shafting 
Heavy Rip 
Rip Saw 
Shafting 
'Shafting 
Shear 


Saw 


motor under actual load conditions without exceeding 
7 amp. per phase per rated hp. This starting device 
must be connected so that motor will take the least 
possible current required to give sufficient starting 
torque and should be equipped with an automatic no¬ 
voltage release which will disconnect the motor in 
case of interruption of circuit. 

In installations exceeding 100 hp., motors under 
7^4 hp. may be connected without starting devices 
provided the starting current does not exceed 14 
amp. per phase per rated hp. 

Where excessive starting torque is required for 
such apparatus as elevators, hoists, air compressors, 
refrigerating machines, fans, high pressure blowers, 
stone crushers, punches, shears or any apparatus with 
heavy flywheel effect, it will probably be necessary to 
install motor of wound rotor type, in order to meet 
the requirements regarding inrush current, or in some 
cases the requirements may be met by a motor of 
squirrel-cage type with a high resistance rotor or a 
motor equipped with friction clutch or loose pulley. 

Over 15 Horsepower 

Motors of over 15 hp. capacity must be of the 
wound rotor type, with external or internal starting 
resistance which will start the motor under actual 
load conditions without exceeding 5 amp. per phase 
per rated hp. In an installation where the motor 
starts without load, a squirrel-cage motor will be 
accepted, provided the starting current does not ex¬ 
ceed that mentioned above for a wound rotor motor. 



















92 


Profitable Power Wiring 


Motor equipment should also be provided with no 
voltage release. 

A good idea from the standpoint of simplicity and 
•order is to take up each motor, one at a time, beginning 
with the smallest. 

Item No. 5—A 1 hp.—1800 r.p.m.—Sump Pump. 

This motor will be thrown across the line with an 
automatic starting switch. Previous chapters have 
stated that actual inrush will be 5 to 6 times full load 
running amperes. Full load amperes are 3, inrush 
will therefore be from 15 to 18 amp. Due to the light 
character of starting, this inrush will be but momen¬ 
tary. An ammeter with a damped movement would 
not register this entire amount, and under ordinary 
circumstances no difficulties will be encountered. To 
allow for starting inrush, multiply full load running 
current by and obtain 3x234, or 734 amp. This 
•calls for No. 14 wire and a 10-amp. fuse. 

Item No. 6—A 3 hp.—1800 r.p.m.—Hydraulic Press. 

This starts a small piece of light jackshafting and 
drives to the loose pulley of the press. The press is 
started by means of a belt shifter that throws the belt 
from the loose to the tight pulley. Under these cir¬ 
cumstances the starting torque is extremely light 
and the motor will come up to speed instantly, so that 
the inrush, while from 5 to 6 times full load running 
amperes, will last but a fraction of a second. Full 
load amperes being 8, inrush will amount to 40 to 
48 amp. The power company’s rules, 14 amp. per 
horsepower, or, in this case, 3 x 14 or 42 amp., are 
permitted, so while there may be some doubt about 
it, under ordinary circumstances it will pass inspec¬ 
tion. Multiplying full load amperes, 8, by 2.5 gives 
20 amp., which means under Code Table C rating that a 
No. 14 wire is ample. 

Item No. 1 A —A 3 hp.—1800 r.p.m.—Rip Saw. 

This motor is directly connected to the saw mandel 
through a belt. The saw operates at a speed of 
3200 r.p.m. This is a severe starting condition and 
constitutes heavy starting duty. Inrush will be just 
as great as in the previous instance, only the dura¬ 
tion will be much longer. The central station may 
complain of excessive starting inrush, but on a large 
installation such as this,, .and for such a small motor, 
it may be allowed. The conductor for this motor 
should therefore be selected for heavy starting duty. 


How Many Power Panels Should Be Used? 93 

To obtain its size, multiply full load amperes by 
3 l / 2 , which is 8x3^4, or 28 amp. From Table C it is 
found that a No. 10 wire is ample. 

Here is an actual instance where different size con¬ 
ductors are used on the same size motors, because of 
different starting requirements, one being light and 
the other heavy. 

Item No. 13—A 5 hp.—1800 r.p.m.—Heavy Rip Saw. 

The motor is belt connected to the saw mandel. 
Owing to the large amount of horsepower on this 
job, the central station will permit this motor to be 
* thrown direct across the line with the use of a start¬ 
ing compensator. Full load amperes are 13, and in¬ 
rush will be from 5 to 6 times this amount, namely, 
64 to 78 amp. Allowable inrush is 5 hp. x 14, or 70 
amp., so that, strictly speaking, it is not within the 
amount permitted by the utility, although in the 
majority of instances they will be passed. As this 
type of load constitutes heavy starting duty, con¬ 
ductors should be designed on a basis of 3J4 times 
full load running amperes, or 13x3j4 equals 45.5 amp. 
This current based upon Table C rating will call for 
No. 8 conductors. 

We now have to consider the larger size motors 
that require special starting devices to limit current 
inrush. 

Item No. 11—A 7j4 hp.—1800 r.p.m.—Motor Gen¬ 
erator. 

Reference to the article in Chapter V, which 
has a table showing the various kinds of starting 
duty, will show that motor generator sets involve 
the lightest possible starting torque. Therefore con¬ 
nect the motor to the lowest voltage tap of the 
compensator. Then starting inrush will not exceed 
full load running amperes. Full load current on this 
motor is 19 amp. Allowing 10 per cent for overload¬ 
ing gives 19 plus 2, or 21 amp. Because there is not 
an excessive inrush, refer to Table A, and accord¬ 
ingly choose a No. 10 conductor, which may carry 
25 amp. continuously. 

Item No. 17—A 7 l / 2 hp.—1200 r.p.m.—Power Shear 
with Heavy Flywheel. 

This motor is connected to the machine through 
spur gearing and the starting torque is extremely 
heavy in proportion to the amount of load carried 
continuously. The manufacturer of the shear fur¬ 
nished the motor with the machine and supplied a 


94 


Profitable Power Wiring 


high resistance silver soldered rotor. This type of 
motor is usually thrown directly across the line. The 
full load running amperes are 20 and starting inrush 
amperes are 65, or slightly more than 3 times full 
load. This inrush is within the limits allowed for the 
standard 5 hp. motor, and will ordinarily be ap¬ 
proved, particularly since it usually takes less than 
one-half of one second for starting. Conductors, 

therefore, may be selected by multiplying the full load 
rating by 2^4, which gives 50 amp. Table C shows that 
a No. 8 wire would be ample. 

Item No. 15—A 10 hp.—900 r.p.m.—Sterling. 

The power company’s rules permit 7 amp. per 

phase per horsepower, or a total inrush of 70 amp. 
Full load amperes are 24. Under these circumstances 
our allowable inrush is slightly under 3 times full 

load running. On the second lowest starting tap 

current inrush is limited to 2$4 times full load run¬ 
ning, or 60 amp., therefore if our motor will start 
the load on this tap, everything will be all right. If 
not, however, it will be necessary to replace it with 
a different type of motor. As the second tap is 
limited to 60 amp., reference to our Table C indicates 
that a No. 6 wire will be required. 

Items 1, 2, 3, 4, 8, 9, 16—A 15 hp.—900 r.p.m.— 
Shafting. 

The rules permit 7 amp. per horsepower, or 15x7 
or 105 amp. for starting inrush. Full load amperes 
for these motors are 35. The second tap limits in 
rush to 2}4 times full load amperes and permits an 
inrush of 2^4x35, or 87*4 amp. On the third tap 
the limit is 3 times full load amperes, giving 3 x 35, 
or 105 amp., which is beyond the limits allowed. The 
conductors, therefore, must be rated for 87.5 amp., 
which from Table C calls for a No. 4 wire. 

Item No. 10—A 20 hp,—900 r.p.m.—Power Brake. 

This machine is a huge thing weighing several 
tons, and whose business it is to take large pieces 
of sheet iron about x A-\n. thick, and 8 ft. long and 
press them into various shapes, such as the tops of 
kitchen ranges, corrugated iron, etc. The motor is 
connected through spur gears to a heavy flywheel, 
and the machine operates through a foot clutch. 
The starting duty is quite severe. The central station 
for this horsepower calls for a wound rotor type and 


How Many Power Panels Should Be Used? 


95 


permits an inrush of 5 amp. per phase per horse¬ 
power, or 100 amp. per phase in this case. The 
full load rating of this motor is 48 amp., so that 
an inrush of lj4 times full load amperes, or 72 amp., 
is well within the limits. It is even possible to 
obtain twice full load starting torque with twice full 
load amperes, or 48x2, which is 96 amp. The feature 
of this type of motor is that the starting inrush 
current may be limited to small amounts and gradu¬ 
ally increased until the motor starts. In that respect 
it is more like the direct current series motor. For 
the purpose here, though, it is safe to figure 1^4 times 
full load running amperes and select the wire for 
72 amp. The requirement of Table C calls for a No. 

5 wire, but No. 4 is adopted because it is more easily 
obtained, at practically little extra expense. No. 4 
wire will carry regularly 70 amp. on Table A rating 
and is therefore ample. 

Item No. 12—25 hp.—900 r.p.m.—Shafting. 

Allowable inrush is 5 amp. per phase per horse¬ 
power or 25 x 5 or 125 amp. Full load amperes are 61 
and as it is possible usually to obtain 1}4 times full 
load torque with \ l / 2 times full load amperes, the in¬ 
rush will be 1^4x61 or 92 amp. The conductor 
from Table C will be No. 3 wire but it is better to 
select a No. 2 because it usually is in stock and costs 
but little more. 

Item No. 7—50 hp.—900 r.p.m.—Compressor. 

Starting duty is severe. Inrush allowable is 5 x 50 
or 250 amp. Full load motor amperes are 113. For 
1 y 2 times full load starting torque, inrush will be 
1*4x113 or 170 amp. Table C shows that a No. 1/0 
wire will serve this purpose, and as its Table C rating 
is 125 amp., which is sufficient to carry a 10 per cent 
overload, it will serve our purpose. 

Nothing has been mentioned about the wiring from 
the controllers to the slip rings of these last three 
motors. As each manufacturer and each size of wound 
rotor motor has a different value for these circuits, it 
has been purposely omitted. In each case it is neces¬ 
sary for the contractor to communicate with the manu¬ 
facturers to obtain this data, fit is always safe, 


96 


Profitable Power Wiring 


however, to figure each of these conductors large 
enough to carry full-load amperes. 

From the preceding data we can begin to prepare our 
motor circuit wiring table. 


Item 

No. 

H.P. 

Motor 

Full 

Load 

Amps. 

Amps. 

Inrush 

Size 

ICircuit 

Ampere 

Size 

Fuse 

Size 

Wire 

Size 

Conduit 
4 wires 

Size 

Motor 

Fuse 

Size 

Motor 

Switch 

Size 

Unfused 

Switch 

5 

1 

3 

18 

30 

15 

14 


10 

30 

30 

6 

3 

8 

48 

30 

20 

14 

V\ 

15 

30 

30 

14 

3 

8 

48 

30 

30 

10 

V 

25 

30 

30 

13 

5 

13 

78 

60 

50 

' 8 

r 

45 

60 

30 

11 

m 

19 

48 

30 

30 

10 

r 

25 

30 

30 

17 

m 

20 

75 

60 

50 

8 

r 

45 

60 

30 

15 

10 

24 

60 

60 

60 

6 . 

l 'A' 

55 

60 

30 

1 

15 

35 

88 

100 

90 

4 

VA" 

80 

100 

60 

2 

15 

35 

88 

100 

90 

4 

1 w 

80 

100 

60 

3 

15 

35 

88 

100 

90 

4 

l W 

80 

100 

60 

4 

15 

35 

88 

100 

90 

4 

1 w 

80 

100 

60 

8 

15 

35 

88 

100 

90 

4 

l w 

80 

100 

60 

9 

15 

35 

88 

100 

90 

4 

1 w 

80 

100 

60 

16 

15 

35 

88 

100 

90 

4 

w 

80 

100 

60 

10 

20 

48 

72 

100 

90 

4 

\W 

80 

100 

60 

12 

25 

61 

92 

100 

100 

2 

2" 

90 

100 

100 

7 

50 

113 

170 

200 

175 

I/O 

2" 

150 

200 

200 

Total... 

.. 237 

562 










Every motor should have a 4-pole, single-throw knife 
switch installed that will permit the opening of every 
wire of the circuit. It is considered good practice to 
install a switch ahead of the compensator, so that the 
circuit may easily be opened to permit adjustments and 
repairs to the compensator if necessary. 

Feeder Circuit Work 

The cost of labor and materials from the motor 
switch through starting devices to the motor remains 
the same, regardless of the kind of wiring installed, so, 
in these calculations that are about to follow, this cost 
is not mentioned. 

Beginning with the service work, the total amperes 
for the entire motor load are 562, to which 10 per cent 


















How Many Power Panels Should Be Used? 


97 


is added, or 56.2 amp. giving a figure at 618 amperes. 
Service work consists in connecting on to the wires of 
the central station and running them into the building. 
Usually this work is done in rigid iron conduit, and in 
this case all four wires of the two phases must go into 
one conduit. Conduit precludes the use of weather¬ 
proof covered wire, and the choice of conductors is 
limited to either varnished cambric or rubber-covered. 
The nearest sizes to 618 amperes are: 

Rubber Covered I Varnished Cambric 

900,000 CM—600 amps. 700,000 CM—600 amps. 

1,000,000 CM—650 amp. I 800,000 CM—660 amps. 

Conduit sizes for four conductors in one conduit are: 


700,000 CM varnished cambric.4^4 in. Pipe 

800,000 CM varnished cambric. Ay 2 in. Pipe 

900,000 CM rubber covered.4^4 in. Pipe 

1,000,000 CM rubber covered.5 in. Pipe 


Since it is desired to give the customer the very best 
kind of a job without spending money unnecessarily, 
the prices should be checked: 


Wire 

700,000 CM VC. 
800,000 CM VC. 
900.000 CM RC. 
1,000,000 CM RC. 

Conduit 

4*4 in. 

5 in. ... 


Price per foot 

. $ 0.66 

. 0.75 

. 0.70 

. 0.76 

Price per foot 

. $ 1.00 

. 1.25 


The best combination is four 900,000 cm. RC. con¬ 
ductors in 4^-in. conduit which will cost $1.70 per foot 
for materials. 

Next consider the service switch. The total am¬ 
perage (allowing for 10 per cent excess) is 618. A 
four-pole, single-throw fused switch is required. Open 
link fuses are recommended because cartridge fuses 
of this size develop excessive heat. Switches are manu¬ 
factured in various standard sizes. There is either the 












98 


Profitable Power Wiring 


400 to 600 amp. size or the 600 to 800 amp. size. In 
this particular instance the maximum amperes for all 
motors operating simultaneously at full load is about 
560. It is seldom that such a current would be drawn, 


Plan view 





Scheme I—All motors Wired Direct from Service 














































































How Many Power Panels Should Be Used? 


99 


and a 600-amp. switch would meet fully all these 
requirements. However, if it is known that in a short 
time the load would be increased, it would be desirable 
as well as good practice to install the larger size, 
800 amp. 

So far the work has been simple and clear-cut, but 
now comes the more difficult problem of wiring layout. 
As the motors are not clearly separated into distinct 
groups, and as they are wide apart, the matter is not 
so self-evident. There are. several schemes which 
differ from the standpoint of cost, flexibility and 
reliability. 

Scheme I—Contemplates installing one main dis¬ 
tribution panel at the service and running separate 
and individual lines of conduit to each motor and 
one spare. 

Scheme II—Includes a combination feeder and dis¬ 
tribution panel at the service, a feeder to A, at which 
point will be introduced another distribution panel. 
The first panel at the service will have separate circuits 
for motors, Items Nos. 1 to 9 inclusive. The second 
panel at A will have separate circuits for motors, 
Items Nqs. 10 to 17 inclusive. 

Scheme III—Includes a combination feeder and 
distribution panel at the service, two feeders to A 
and B at which points distribution panel boards will 
be mounted. The panel board at the service will 
have two separate feeder gaps to A and B and will 
have separate circuits for motors, Items Nos. 1 to 9 
inclusive. The panel board at A will have separate 
circuits for motors, Items Nos. 10 to 14 inclusive, the 
panel board at B will have separate circuits for motors, 
Items Nos. 15 to 17 inclusive. 

Scheme IV—Includes a feeder panel only at the 
service. Four feeders from this point run to A, B, C 
and D. At A will be a panel controlling motors Nos. 

10 to 14 inclusive; at B a panel controlling Nos. 15 to 
17 inclusive; at C a panel controlling Nos. 1 to 4 
inclusive, and at D a panel controlling Nos. 5 to 9 
inclusive. 


100 


Profitable Power Wiring 


For the interest of the reader all detailed description 
leading up to the assembling of the panel board, includ¬ 
ing feeder conduit and wire data for the various 


PLAN VIEW 





Scheme II—Combination Feeder and Distribution 
Panel at Service with Additional Board at A 













































































How Many Power Panels Should Be Used? 101 

schemes have been purposely omitted. It is the writer’s 
opinion that scheme No. 4 is the best for this installa¬ 
tion. After presenting the comparative costs of the 
different schemes it will be shown how such an equip¬ 
ment is to be designed. 

Only that portion of the work which includes the 
main feeder or distribution panel, the several sub¬ 
feeders that run to the sub-distribution panels, and the 
motor circuit work that terminates at the motor switch 
or motor starter will be considered because the service 
wiring or the installation of motors, motor control and 
motor circuits from the motor switch to the motors 
themselves will be identical regardless of what scheme 
may be finally adopted. 

The prices of materials, such as panel boards, conduit 
and wire, are the quotations at the time this is written, 
and while they may not be exactly right at the time this 
is read, relatively the proportions will always remain 
the same. The same applies to labor costs. 

In the preparation of these costs for the different 
schemes, conduit, wire, panel boards and fuses were 
included. Such fittings as elbows, separate couplings, 
lock nuts and bushings, straps and screws were pur¬ 
posely omitted, as it was the intention to obtain only 
comparative figures. In computing labor costs, the 
costs of erecting conduit, pulling wires, mounting and 
connecting panel boards, were assembled for each 
scheme. 

It would be useless and of little value to the reader 
to publish the various estimates, computations and 
various pricing and extensions that were involved in 
preparing these data. The following results have 
been tabulated for inspection and analysis. 

The writer is frank to acknowledge that the results 
are remarkable and unique. In his opinion they offer a 
splendid opportunity for interesting discussion in any 
meeting of electrical contractors or engineers. 


102 


Profitable Power Wiring 

Estimated Costs for Various Wiring Schemes 


Scheme 
Panelboards 
Conduit 
Wire . 


No. 1 
$ 335.00 
387.00 
558.00 


No. 2 
$ 410.00 
284.00 
445.00 


No. 3 No.4 
$ 405.00 $ 575.00 

329.00 298.00 

556.00 530.00 


Total Costs—Material.. 
Total Costs—Tabor.... 


.$1,280.00 $1,139.00 
. 598.00 442.00 


$1,290.00 $1,403.00 

493.00 467.00 


Total Costs—Material & Labor. $1,878.00 $1,581.00 


$1,783.00 $1,870.00 


The maximum difference is between scheme No. 2 
and‘scheme No. 1, and amounts to $297. 

One might be tempted to say, “Why all this bother 
about preparing cost prices ?” And the answer is that, 
after all is said and done, whatever method or scheme 
for wiring is adopted it must, in addition to rendering 
perfect electrical service, be reasonable in cost. No 
scheme, no matter how perfect, is worth consideration 
if its original cost is too high. The main purpose of 
all that has been said before is not how cheap can this 
work be done, but how much can we put into a job to 
give the best electrical service, and at a cost that will 
not be beyond competition And an examination of 
the figures compiled above will bring out many inter¬ 
esting features, well worth consideration. 

Scheme No. 1, which would never be installed by any 
contractor who is familiar with power-wiring equip¬ 
ment, proved to be the most expensive of any. 

Scheme No. 4, which is a spendidly designed system 
of wiring, proved to be practically as expensive as the 
absurd sceheme No. 1. 

Scheme No. 2, in which the main panel was cut 
down and a separate feeder run to one other panel, 
proved to be practically $300 cheaper than scheme 
No. 1. 

Scheme No. 3, where there was one additional panel 
over the two in scheme No. 2, became at once $200 
more costly, and by putting in a feeder panel and four 

















How Many Power Panels Should Be Used? 103 


distribution power panels as in scheme No. 4 our cost 
jumped up another $100. 

An analysis of panel board costs will be interesting. 


5™ FLOOR 
BLDG. "A" 


FLOOR 
BLDG. "A" 


GROUND OR 
is FLOOR 


Scheme III—Combination Feeder and Distribution 

Panel at Service with Additional Boards at A and B 

For scheme No. 1, consisting of one large distribution 















































































104 


Profitable Power Wiring 


panel, the cost was $335. In scheme No. 2 half the 
motor circuits were taken off, leaving a combination 
feeder and distribution panel at the service and another 
distribution panel in the center of the place. This com- 


PLAN VIEW 


5“ FLOOR 
BLDG. "A" 


FLOOR 
BLDG. "A" 




Scheme IV—Feeder Panel Only at Service with Four 
Distribution Panels at A, B, C and D. 
















































































How Many Power Panels Should Be Used? 105 

bination cost $410. In scheme No. 3 a combination 
panel at the service consisting of two feeder circuits 
and about 9 motor circuits and then two distribution 
panels cost altogether $405, or about the same as 
scheme No. 2, $410. In scheme No. 4 a straight feeder 
panel at the service and the four separate distribution 
panels jumped the cost up to $575. Any further subdi¬ 
vision of circuits and increase in the number of panels 
would correspondingly raise the panel board costs. 

To put it roughly, in the form of a rule based on 
practical experience, it is uneconomical to put in such 
a number of panels that the cost of these panel boards 
and feeders is in excess of the cost of individual cir¬ 
cuits. Like every other known rule, it has its excep¬ 
tions, which will be discussed later. 

Similar analyses might be made from the table by 
the reader for conduit, wire and labor. 

Having described briefly the different costs for the 
different schemes, it will be interesting to discuss them 
from the angle of serviceability, namely: 

1. Continuity of operation 

If fuses and conductors are properly selected, and 
of ample capacity an uninterrupted supply of power 
is secured. 


2. Flexibility 

If it is desired to replace a motor with one of next 
larger size, it is possible to do so if fuse gaps have 
been ample in capacity. If it is desired to install an 
additional motor, it may be done without running all 
the way to the service, by connecting to the nearest 
panel. If it is desired to rearrange motor locations 
from one floor of the building to another, it may be 
done with less expense if ample capacity is provided 
in the first installation. 

Comparing the various schemes on this basis throws 
out scheme No. 1, because it is the most expensive, and 


106 


Profitable Power Wiring 


it has absolutely no provisions for future expansion or 
additions. For every new motor added, it requires 
an expensive long circuit run and a very difficult and 
mean job at the service to introduce a new fuse cutout. 

Scheme No. 2 has distinct merit because it is the 
cheapest. It has the disadvantage of scheme No. 1, 
although not to the same degree. For every new motor 
added, a run must be made either to the service or to 
the sub-distribution panel, although in this instance the 
runs are not so long. 

Scheme No. 3 is not only more expensive than 
scheme No. 2 to the extent of $200, but it has not 
sufficient additional capacity to warrant this extra cost. 

Scheme No. 4 is $300 more costly than scheme No. 
2, but it has these advantages: Provisions at the 
service allow for a new separate feeder circuit at any 
time, four distribution panels and at every one of these 
are provisions for an additional circuit; a new group 
of motors or new machinery may be installed in almost 
any section of the shop and still be near enough to a 
distribution point to involve only a small wiring cost. 
This scheme has the greatest allowance for additional 
equipment at the lowest wiring cost. 

The choice, therefore, narrows down to between 
schemes Nos. 2 and 4. If it were a case where compe¬ 
tition was involved and the customer could not and 
would not see the advantages offered by scheme No. 4, 
the contractor would be justified in adopting scheme 
No. 2, for it will work perfectly. If, however, the 
customer could be made to appreciate the value to him 
of scheme No. 4, that by all means would be an ideal 
solution of this example of wiring. 

Having decided that the best job is the one to be 
installed, there follows a detailed description of how it 
is calculated and designed: 

There will be required one main feeder panel at the 
service, and four distribution panels at A, B, C and D 


How Many Power Panels Should Be Used? 107 


CIRCUITS REQUIRED ON FOUR DISTRIBUTION 
PANELS FOR SCHEME IV. 

Motor 

A 

Four Pole 


B 


Item 

Circuits 

Motor 


Four Pole 

No. 10 

100 Amp. 

Item 


Circuits 

" 11 

30 " 

No. 15 


100 Amp. 

“ 12 

100 " 

“ 16 


60 “ 

“ 13 

60 M 

“ 17 


100 “ 

“ 14 

30 “ 

Spare 


100 “ 

Spar' 

100 •• 







Price of panel 

$80 


Price of panel $95 


p 



C 

Motor 


Four Pole 

Motor 

Four Pole 

Item 


Circuits 

Item 

Circuits 

No. 5 


30 Amp. 

No. 1 

100 *• 

“ 6 


30 “ 

“ 2 

100 “ 

“ 7 


200 " 

“ 3 

100 “ 

“ 8 


100 44 

“ 4 

100 “ 

“ 9 


100 “ 

Spare 

100 “ 

Spare 


100 “ • 


Price of panel $110 


Price of panel $120 

•NOTE—The reason 100 amps, was selected for the space in panel 

D instead of 200 amps, was that any 

load requiring 200 amps, additional 

would 

overload the feeder and in that case such a gap would 

be useless. 


requiring the circuits as tabulated. It is advisable to 
make the spare the same in size as the largest circuit. 

The next step is to determine the amount of current 
required by each feeder. The feeder running to A will 
carry current for motors Items 10 to 14 inclusive. Re¬ 
ferring to the table earlier in the article for full load 
currents, it is found that these motors take a total of 
149 amp. Excess inrush for the largest motor, item 
No. 12 is 31 amp., so that total feeder capacity is 149 
• plus 31 or 180 amp. The fuse gap must accordingly 
be designed for 200 amp. 

The feeder running to B will carry current for 
motors, items Nos. 15 to 17 inclusive, and the table 
for full load currents shows a total of 79 amp. for 
them. Excess inrush for the largest motor item No. 
16 is 53 amp. Total feeder capacity, therefore, is 79 










108 


Profitable Power Wiring 


plus 53 or 132 amp., and the fuse gap must be designed 
for 200 amp. 

Feeder C will feed motors 1 to 4 inclusive, which 
take a total of 140 amp. Excess starting inrush of the 
largest motor is the difference between 88 and 35 or 53 
amp. Total feeder capacity, therefore, is 140 plus 53 
or 193 amps., thus requiring a 200-amp. gap for 
feeder C. 

Feeder D supplies power for motors Nos. 5 to 9 in¬ 
clusive, which take 194 amp. Excess starting inrush 
of the largest motor, namely the 50 hp., is the differ¬ 
ence between 170 and 113 or 57 amp. One feeder, 
therefore, must be large enough to carry 194 plus 57 
or 251 amp. The fuse gap will therefore be 400 amp. 

The spare ought to be around 200 amp. rather than 
400, because an additional load of 400 amp. would be 
far in excess of the ability of the service switch or 
cables to carry it. 

The feeder panel will therefore be designed for four 
200-amp., 4-pole gaps and one 400-amp., 4-pole gap. 
The cost will be $170. 

The total panel cost for scheme No. 4 will be: 


Panel A. 


...$ 95 

Panel B . 


. 80 

Panel C. 


. 110 

Panel D. 


. 120 

Feeder panel. 


. 170 

Total. 


...$575 


The next job is to figure out what the various feed- * 
ers will require in the way of conduit and wire. 

Feeder A will require 180 amp. and Feeder B 132 
amp. While there may be a temptation to use 3/0 in 
the first case, which is rated to carry 175 amp., and use 
No. 1/0 in the second instance, which is rated at 125 








How Many Power Panels Should Be Used? 109 

amp., it is not considered good practice, and the writer 
would recommend using the next larger sizes, namely 
4/0 and 2/0, for, in nine out of ten cases, the load will 
be increased. These feeders will therefore consist of: 

Feeder A...Four No. 4/0 RC in 2^4-in. conduit 

Feeder B.Four No. 2/0 RC in 2^-in. conduit 

Feeder C carries 193 amp. and Feeder D, 251 amp. 

Feeder C will therefore consist of four No. 4/0 RC. 
conductors in 2l4-in. conduit. Feeder D will consist of 
four No. 4/0 V.C. conductors in 2^-in. conduit. 




CHAPTER EIGHT 


Economy of Grouping Motor Circuits 
in One Conduit 


I N common with previous chapters in this book, the 
present chapter which relates to layout for three 
phase motor wiring will be based on a practical exam¬ 
ple. In this case it will be a wood box manufacturing 
plant with a cellar and first floor to use twenty motors 
from 220-volt, 3 phase, 60-cycle, 3-wire service. 

Not only will calculations be given for conductors, 
distribution system, feeders, and panels as previously 
but in addition particular emphasis will be given to 
the matter of economy by running several motor cir¬ 
cuits in one conduit instead of individual conduits. 
The cost will first be worked up for a separate conduit 
for each circuit and then comparison will be made for 
the installation of several circuits in one larger conduit. 

The following list describes the motors and their 
particular drives, and at the same time the individual 
circuit wiring which will be necessary for each of 
these motors is included. 

Item 1. 5 HP 1200 RPM—Shafting to two conveyors, 

each controlled by shifting belt from loose to tight pulley. 
This drives a light piece of shafting and pulleys at a 
moderate speed upon starting, so that starting amperes 
can be estimated at 2^ times full load running amperes, 
which will be 15 x2^ or 37.5 amps. On the Table C 
rating use three No. 8 wires in 1 in. conduit protected by 
a 35 amp. fuse. 



112 


Profitable Power Wiring 



Item 2. 5 HP 1200 RPM—Shafting to three groovers, 
each controlled by shifting belt from loose to tight pulley. 
This is identical with item No. 1. 

Item No. 3. 15 HP 900 RPM to friction clutch pulley 
on shaft for ten circular rip saws. This motor starts 
under the very lightest starting conditions and hence may 
be started from the first or second lowest starting tap of 
the compensator. On compensators for this size motor, 
the first tap is a 50 per cent reduction in line voltage or 
an inrush corresponding to \ l /2 times full load full amperes. 
In this instance full load amperes are 40 and inrush will 
be 40xlj4 or 60 amps. Therefore use three No. 6 wires 
in 1*4 in. conduit. 

Item No. 4. 20 HP 900 RPM to macerator direct 
through belt. This motor operates a machine for making 
sawdust from shavings and small pieces of wood which 
operates at a speed of 1000 RPM and hence starting condi- 














Economy of Grouping Circuits 


113 


tions will be moderately severe. Connect to the second 
compensator tap. This limits inrush to 2^4 times full 
load amperes or 52x2*^, which is 130 amps. Therefore 
use three No. 1 wires in lj4-in. conduit. 

Item No. 5. 5 HP 1800 RPM shafting to three swing 
cross cut saws. This motor is belted to shafting. Since 
all of the machines are started at the same instant as the 
motor and there are no clutches or tight and loose pulleys, 
starting conditions are severe. As this motor is thrown 
directly across the line without a compensator, to obtain 
the size of conductors multiply full load running amperes 
by 3j4 or 15 x3}4 or 52 amp. inrush. Table C rating for 
a No. 8 conductor is 50 amps, which in this instance will 
serve satisfactorily. These three conductors are to be 
put in a 1-in. conduit. 



Item No. 6. 5 HP 1200 RPM to shafting to three swing 
cross cut saws. This is identical with previous item No. 
5, therefore we use the same equipment. 














114 


Profitable Power Wiring 


Item No. 7. 50 HP 900 RPM to shafting to two planers, 
each started by taking up slack of the belt with an idler 
pulley arrangement. This is a squirrel cage motor and 
drives 20 ft. of shafting with three hangers and three 
pulleys. Two planers are operated from the pulleys by 
shifting an idler pulley, thus taking up the slack of the 
loose belt and putting them in motion. Starting condi¬ 
tions are the lightest possible; therefore start on the 
lowest compensator tap. This connection limits the start¬ 
ing inrush to full load amperes which in this instance is 
125 amps. Select a conductor for 10 per cent excess 
capacity or 137.5 amps. The nearest to this is a No. 2/0 
R. C. which normally carries 150 amps. Three of these 
will go comfortably in a 2-in. conduit. 

Item No. 8. 50 HP 900 RPM (slip ring) belted direct 
to 50 in. band resaw. This type of machine calls for the 
heaviest starting duty, namely from 200 to 300 per cent 
starting torque. The slip ring type of motor is eminently 
fitted for this kind of work and will produce a maximum 
of starting torque for a minimum of starting amperes 
inrush. Standard control for this type of motor is de¬ 
signed to limit inrush to 1^2 times full load amperes. 
Full load is 130 amps., so that inrush will be 130xlj4 or 
195 amps. On Table C rating this would take a No. 1/0 
R. C. which may be fused to 200 amps.; but as the normal 
carrying capacity is only 125 amps., it is suggested that 
the next larger size, namely 2/0 R. C., be used which will 
carry continuously 150 amps, and may be fused to 225 
amps. Three of these wires will accommodate themselves 
agreeably in a 2-in. conduit. 

Item No. 9. 20 HP 1800 RPM to exhaust fan, squirrel 
cage. This is the very lightest kind of starting duty, 
therefore use lowest starting tap of the compensator. 
Starting inrush there will be equivalent to full load am¬ 
peres, which in this particular instance will be 52 amps. 
Allowing 10 per cent additional, there is required a con¬ 
ductor for 57.2 amps., which for table A rating means 
No. 4 R. C. Three of these are permitted in one 1%-in. 
conduit. 

Item No. 10. 20 HP 1800 RPM (squirrel cage) operat¬ 
ing an exhaust fan. As this item is identical with the 
previous one, use the same equipment. 

Item No. 11. 10 HP J200 RPM to multiple rip saw. 

It is connected through a belt direct to the saw mandril 


Economy of Grouping Circuits 


115 


which has on it from two to five circular rip saws. The 
starting duty is moderate when the size of the motor is 
taken into consideration, and the apparatus will therefore 
start easily on the second tap which will take, roughly, 
about 2 to 234 times full load amperes. Running amperes 
are 28. Inrush will be from 50 to 70 amps. A No. 6 wire 
may be fused to 70 amps, and will suffice for this circuit. 
Therefore allow for three No. 6 DBRC stranded wires 
in l^-in. conduit. 

Item No. 12. 35 HP 600 RPM direct to 45-in. band 
resaw. This is a slip ring motor direct coupled to the 
shaft operating the wheel on which the saw band rides. 
Under these conditions, starting conditions are of the 
severest. A motor of this type, as explained before, will 
develop maximum torque for minimum inrush so it is 
safe to estimate on 134 times full load amperes. Full load 
amperes are 90 so that inrush will be 90 x 1*4 or 135 amps. 
A No. 1 wire may be fused up to 150 amps., and as its 
running current capacity is in excess of 110 per cent of 
the motor rating, it is permitted. Three No. 1 will fit in 
a 134 -in. conduit. 

Item No. 13. 7.5 HP 1200 RPM to shafting, operating 
some light machines through tight and loose pulleys. 
Starting conditions are very light and the motor will 
easily start the load on the second compensator tap, if 
not the first. Inrush will be from 2 to 234 times full load 
running amperes which are 22. Inrush amperes will 
range between 44 and 55. Three No. 8 wires which may 
be fused to 50 amps, will serve this condition in 1-in. 
conduit. 

Item No. 14. 5 HP 1200 RPM to shafting, nailers 
operated by clutch pulleys. This motor is started directly 
across the line without any compensator and the momen¬ 
tary inrush amperes will be from 5 to 6 times the full 
load amperage of 15, namely 75 to 90 amps. The starting 
demand is light, and therefore the peak demand will be 
of but brief duration so it is permissible to multiply the 
full load amperes by 234, giving 3734 amps. This would 
demand three No. 8 wires fused up to 50 amps, in 1-in. 
conduit. 

Item No. 15. 5 HP 1200 RPM to shafting, nailers oper¬ 
ated by clutch pulleys. This is identical with the previous 
item. 


116 


Profitable Power Wiring 


Items Nos. 16, 17, 18 and 19. 3 HP 1800 RPM motors 
belted direct to four swing cross cut saws, which are run 
at a speed of about 2800 RPM. This starting condition 
is severe. Therefore multiply the full load amperes, 9, by 
3^4 and obtain 31}4 amps. Three No. 10 wires ordinarily 
would serve, but in this instance, it would be better to 
use 3 No. 8 wires in 1-in. conduit fused no more than 
35 amps. 

Item No. 20. 3 HP 900 RPM to printing press. This 
motor is connected direct to a loose pulley and starting 
duty will be light. Under these circumstances, multiply 
full load amperes, 9, by 2^4 and obtain 22*4. Three No. 
12 wires fused to 25 amps, would serve nicely in a ^-in. 
conduit. 



□ MOTORS ON FIRST FLOOR 

Composite Cellar and First Floor Plan 

In order to decide on the number of distribution 
panels, boxes and the arrangement of feeders, a con- 














Economy of Grouping Circuits 


117 


venient method to represent the motors in the cellar 
and on the first floor, would be to show the first floor 
motors by solid blocks and the cellar motors by dotted 
blocks. The illustration shows such a diagram before 
showing the wiring. An inspection of the diagram 
indicates clearly that there are two distinct groups. 

Group A would comprise those items in the lower 
left hand corner, namely, Nos. 1, 2, 3, 4, 16, 17, 18, 
19 and 20. 

Group B would comprise the remaining items, 
namely, Nos. 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15. 

There is therefore required a combination feeder 
and distribution panel for the motors in Group A, and 
a distribution panel for the motors in Group B. 

Service enters in the cellar where a transformer 
vault has been constructed. The low tension service 
will leave the vault from the side. The panel can be 
mounted directly over the conduit leading out. This 
panel, designated as A, will consist of a main low 
tension fused service switch, a feeder gap for Group 
B motors, a spare feeder gap, individual fuse gaps for 
items 1, 2, 3, 4, 16, 17, 18, 19 and 20 and a spare motor 
fuse gap. Panel B will contain separate fuse gaps 
for items 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and one 
spare. 


Arrangement of Motors 



Group A 



Group B 


Item 

H.P. 

Size Gap 

Item 

H.P. 

Size Gap 

1 

5 

60 

5 

5 

60 

2 

5 

60 

6 

5 

60 

3 

15 

100 

7 

50 

200 

4 

20 

200 

8 

50 

200 

16 

3 

60 

9 

20 

100 

17 

3 

60 

10 

20 

100 

18 

3 

60 

11 

10 

100 

19 

3 

60 

12 

35 

200 

20 

3 

30 

13 

7.5 

60 




14 

5 

60 




15 

5 

60 

Spare 

.... 

200 

Spare 


200 

Total 

70 


Total 

212.5 



118 


Profitable Power Wiring 


The total horsepower equals 70 plus 212.5 or 282.5 
hp. Total full load running amperes are approximately 
three times total horsepower or 847.5 amps. This will 
require two conductors for each leg. 

It is advisable to allow extra capacity on this short 
run through the transformer vault wall. It is there¬ 
fore recommended that six 600,000 C.M. R.C. conduc¬ 
tors be used, two in multiple for each leg. The con¬ 
ductors terminate at the fused end of our main service 
switch on Panel A. An 800-amp. switch is not quite 
large enough so use the next size, namely 1200 amp. 
3-pole link-fused. 

In addition there should be a feeder circuit for 
Panel B. This controls a total of 212.5 hp. or a 
demand of 637.5 amps. Allow for 10 per cent addi¬ 
tional when selecting the feeder which is 637.5 plus 
63 or 700 amps. Three 1,100,00 D.B.R.C. conductors 
in 4p 2 in. pipe or three 900,000 C.M. varnished cam¬ 
bric in 4 in. pipe, will serve this purpose. If two 
conductors per leg are run use either six 500,000 
D.B.R.C-. conductors in 4p 2 in. pipe or six 350,000 
C.M. varnished cambric conductors in 4 in. pipe. 

The relative costs per running foot of conduit are 
as follows: 

3 ft. No. 1,100,000 CM DBRC conductor at $1.25 per ft..$3.75 
lft. in. conduit .. 1.10 

$4.85 

3 ft. No. 900,000 CM varnished cambric conductor at 

$1.00 per ft.....$3.00 

1 ft. 4 in. conduit at 86 cents per ft....86 

$3.86 

6 ft. No. 500,000 CM DBRC conductor at 45 cents per ft..$2.70 
1 ft. 4^2 in. conduit at $1.10 per ft. 1.10 


$3.80 









Economy of Grouping Circuits 119 

6 ft. No. 350,000 CM varnished cambric conductor at 46 

cents per ft.,...$2.76 

1 ft. 4 in. conduit at 86 cents per ft.86 


$ 3 . 62 . 



Wiring Layout for Grouped Circuit Runs 


Of the above four alternatives, all of which will 
perform the same duty, the last scheme is selected on 
account of lowest cost, which is $1.23 less than the 
first plan and $0.24 and $0.18 less than the second 
and third plans respectively. 

The fuse gap will be great enough to protect this 
feeder and will be from 600 to 800 amps. The spare 
feeder gap may be anything one chooses and 400 amps, 
is about the right one to be used. 


























120 


Profitable Power Wiring 


The following gaps are required for the motors and 
feeders: 

Item 20 .1- 30 amp. 3 pole 

Items, 1, 2, 16, 17, 18, 19.6- 60 amp. 3 pole 

Item 3 .1-100 amp. 3 pole 

Item 4 and spare motor. 2-200 amp. 3 pole 

Spare feeder .1-400 amp. 3 pole 

Feeder to B .1-800 amp. 3 pole 

Main switch ...1-1200 amp. 3 pole fused 

The main bus bars must be of sufficient capacity to 
carry 1200 amps, and gradually taper down. 

Terminal lugs should be at the end where the service 
wires enter and far enough away to permit a gradual 
bend of the conductors. They should be multiple lugs, 
for two conductors. Lugs at the circuit end of feeder 
gap should also be multiple lugs. 

Panel B should contain the following gaps: 

Items 5, 6, 13, 14 and 15.5- 60 amp. 3 pole 

Items 9, 10 and 11.3-100 amp. 3 pole 

Items 7, 8, 12 and spare.4-200 amp. 3 pole 

Next take up the circuit runs. First prepare a table 
showing wire sizes and conduit for each item. 

Wire and Conduit Sizes for Circuit Runs 


:m No. 

H.P. 

20 __ 

3 

16, 17, 18, 19. 

. 3 

1, 2, 5, 6, 14, 15.. 

_ 5 

13 _.. .... 

. 7y 2 

11 __ 

. 10 

3 ... 

IS 

4 .... 

_ 20 

9, 10 . 

.. 20 

12 . 


7 8 .... 

__ 50 


Conduit Size 

1 X 
1 
1 

154 

154 

in 


Wire Size 
12 
8 
8 
8 
6 
6 
1 
4 
1 

2/0 


The diagram shows a layout with individual con¬ 
duit runs, each containing three wires of the size 
specified in the accompanying table. The rules and 



















Economy of Grouping Circuits 121 

regulations, however, permit the use of nine wires per 
conduit and under these circumstances the circuits can 
be run as shown below. 

We begin with Items 1, 2 and 20. 

Item 1 has 3 No. 8 wires in 1 in. conduit. 

Item 2 has 3 No. 8 wires in 1 in. conduit. 

Item 20 has 3 No. 12 wires in in. conduit. 

By referring to the code table it is noted that nine 
No. 8 wires are permitted in 1% in. conduit so that 
this combination will work. 



Wiring Layout for Individual Circuit Runs 

Hence the run will start from the panel in 1% in. 
conduit to A where there is a junction box or a T fit¬ 
ting from which three wires are brought out to the 





































122 Profitable Power Wiring 

switch for item 20 in 24 in. conduit. From there the 
run will have six No. 8 wires which go in 1*4 in¬ 
conduit, to B where another T fitting is used. From 
there two runs of three No. 8 wires in 1 in. conduit 
continue to the other two motors. 

In the next combination are items 3, 4 and 16, for 
which the number and sizes of conductors used for 
them are given as follows: 

Item 3 has 3 No. 6 wires in 1 % in. conduit. 

Item 4 has 3 No. 1 wires in in. conduit. 

Item 16 has 3 No. 8 wires in 1 in. conduit 

To solve this condition with sufficient exactness so 
that it will be approved, we proceed as follows: 

Cross Section Area of Conductors 

No. 8 SBRC each .055 x 3 equals.165 square inches 

No. 6 DBRC each .166 x 3 equals.498 square inches 

No. 1 DBRC each .264 x 3 equals.792 square inches 

Total area of conductors_ 1.455 square inches 

A rule states that area of conductors should not 
exceed 40 per cent of area of conduit. Forty per cent 
of 2 in. conduit area equals 1.257 sq. in. We must 
accordingly use the next size which is 2^4 in. conduit. 

Run from the panel box to C, one line of 2^4 in. 
containing three No. 1, three No. 6 and three No. 8 
wires. At C put in a junction box or T and continue 
three No. 8 in 1 in. conduit to switch for Item No. 16. 

The remaining run is for items 17, 18 and 19. Each 
of which is a 3 hp. motor and requires three No. 8 
wires in 1 in. conduit. The code table shows that nine 
No. 8 wires may be put into 1*4 in. conduit. That 
size is run to D where a T is placed and six No. 8 
wires are continued to. E in 1^4 in. conduit. The 
separate branches have three No. 8 wires in 1 in. 
conduit. 







Economy of Grouping Circuits 


123 


Next come the runs from the other panel. The first 
serves items Nos. 5, 6 and 15 which are 5-hp. motors, 
each requiring three No. 8 wires. Nine of those will 
go into 1^4 in. conduit. As the switches are all located 
on one column, a junction box at the terminal would 
be best. 

Next in line are item Nos. 9, 10 and 11. These call 
for six No. 4 wires and three No. 6 wires. If nine No. 
5 wires will go into 2 in. conduit, then six No. 4 and 
three No. 6 wires will fish quite nicely in 2 in. conduit. 
We therefore use 2 in. conduit up to F where the three 
No. 6 wires come out and continue six No. 4 wires in 
2 in. conduit from there. 

The next run is for item Nos. 7 and 13. These call 
for three No. 2/0 and three No. 8 wires. It is a short 
run and while 2 in. conduit will carry all six wires it 
is best to follow the regulation and use 2in. 

Having shown how conduit can be advantageously 
laid out to save both labor and material, two estimates 
have been prepared to show just how the two types of 
construction would compare. There is no saving in 
wire cost, but only in conduit and the labor of instal¬ 
ling it, which is offset by the extra cost of fittings. 
The estimates therefore consider only the labor, con¬ 
duit and conduit fittings up to the motor starting 
switch. The amount of wire remains the same and 
the cost of pulling it in is about equal. Those motor 
runs that are individual will not be considered in the 
table because their cost remains the same. 

It is plain and clear that there is a distinct saving by 
grouping three circuits of three wires each in one con¬ 
duit. The saving in this instance is somewhat over 
33 1/3 per cent or $169. This method of construction 
is perfectly proper and is approved in every sense. It 
makes for a neat, clean job and has proved both reli¬ 
able and satisfactory. 


124 


Profitable Power Wiring 


Comparison of Material and Labor for Two Schemes of 
Construction 


Separate Conduits 
Feet 


in. Conduit . 20 

1 in. Conduit . 510 

1*4 in* Conduit . 305 

\y 2 in. Conduit .—. 60 

2 in. Conduit . 40 


Number 

1 in. Elbow and Coupling. 30 

1*4 in. Elbow and Coupling. 12 

lyi in. Elbow and Coupling. — 

2 in. Elbow and Coupling. 3 

2J4 in. Elbow and Coupling.. — 


Number 

1 in. Junction Box—. — 

1% in. Junction Box- — 

2 in. Junction Box. — 


Material Cost . $146.00 

Eabor—Hours per man at $1.00 300.00 


Group Conduits 
Feet 


85 

105 

120 

50 

Number 

1 

4 

3 

3 

Number 

1 

5 
2 

$97.00 

180.00 


Total Costs .. $446.00 $277.00 

This ends our problem for alternating current motor 
wiring, 3 phase, 220 volts, 60 cycles. 


Other Systems 

There still remain other alternating current systems 
of wiring that have not yet been discussed. These are: 

1. Two-phase, 220 volt, 60 cycle, 3 wire. 

2. Two-phase, 440 volt, 60 cycle, 4 wire. 

3. Two-phase, 550 volt, 60 cycle, 4 wire. 

4. Three-phase, 440 volt, 60 cycle, 3 wire. 

5. Three-phase, 550 volt, 60 cycle, 3 wire. 

6. In addition there are alternating current systems 
of 25, 40 and 50 cycles. 

7. Then there are high tension systems, where 
motors are operated direct from 2200 volts, 2 and 3 
phase, 25 and 60 cycles. 

8. A frequent condition arises where it is desired to 
use 2-phase motors on 3-phase service. 


















Economy of Grouping Circuits 


125 


A few brief notes on each of these follow: 

1. For 2-phase, 220 volt, 60 cycle, 3 wire. In this 
type of system, the two ends of each of the phases are 
tied or connected together as a common neutral wire. 
The current flow in this common neutral is exactly 
1.41 times the current in either of the other two out¬ 
side conductors. That means that throughout the 
entire wiring system, the neutral conductor must be of 
such size that it will carry 1.41 times the current car¬ 
ried by either of the outside wires. Beginning at the 
service, the neutral wire is heavier; the neutral bus 
bar on the feeder panel and distribution panel is 
heavier, and any 3-pole switch used must have a center 
leg that is great enough to carry this excess current. 
The fuse at the ultimate branching point must also be 
heavy enough for this excess of 1.41 times the am¬ 
peres of either outside leg. Where the outside amperes 
are 30, the neutral amperes will be 30 x 1.41 or 42.3 
amps. Therefore, the switch must be a 3 pole, 60 amp. 
size. Fuses must be approximately 30 amp. on the 
outsides and 45 on the neutral. This is accomplished 
either with a special switch, made to order, or a 
separate switch and three single pole main line cutouts. 
Bearing these facts in mind, one can proceed with the 
wiring design in the same fashion as described in a 
previous example for two-phase motors. 

2. Two-phase, 440 volt, 60 cycle, 4 wire. 

Wiring for a system of this kind is worked out just 
the same as described in the example for two-phase, 
220 volts except that the ampere rating of the motors 
is but one-half of that demanded for 220 volts. It is 
well to bear in mind special spacing for panelboard 
bus bars, fuses and live parts, and special swtiches, 
fuses and other accessories. 


126 


Profitable Power Wiring 


3. Two-phase, 550 volt, 60 cycle, 4 wire. 

The design wiring for this system is the same as 
that described in the chapter on two-phase wiring 
except that the amperes are but two-fifths of what 
they are at 220 volts. At the same time special 
spacing rules should be observed for panel bars. 
Special fuses and special switches are required. 

4 and 5. Three-phase, 440 and 550 volt, 60 cycle, 
5 wire services. 

These wiring systems are designed in line with the 
procedure illustrated in the example for three-phase 
motors, except that for 440 volts the amperage is but 
half of that at 220 volts and at 550 volts it is two- 
fifths of that at 220 volts. Again, attention is called 
to the fact that special spacing on panel boards and 
switches, and special fuses are required. 

6. The matter of frequencies, whether 25, 40, 50 
or 60 cycles, does not afifect the wiring in any respect. 
It does, however, have an important bearing on all 
solenoids, trip coils, compensators or any other appa¬ 
ratus required for protection or starting purposes. 

7. No mention has been made of the wiring of 2200 
volt apparatus as that type of installation is less com¬ 
mon and therefore of less general interest. 

8. There is one other condition that comes up on dif¬ 
ferent occasions, namely the use of two-phase motors 
on three-phase service or vice versa. There are two 
ways in which this may be accomplished: 

(a) An autotransformer 

(b) Two Scott connected transformers. 

The autotransformer costs approximately one-third 
as much as the two Scott connected transformers, but 


Economy of Grouping Circuits 


127 


its use is not recommended. The reason for this is 
that a ground on the wiring on either side of the auto¬ 
transformer causes a dead short circuit on the trans¬ 
former and much trouble is caused thereby. The use 
of two Scott connected transformers is a safe and 
sound procedure. 

The question as to air or oil cooled depends mainly 
upon local conditions. 















